Boynton's greeting card designs for Recycled Paper Greetings were at the forefront of the Alternative Cards commercial movement that began in the mid-1970s. According to RPG co-founder and president Mike Keiser, over 200 million copies of Boynton's distinctive humorous cards...featuring an assortment of unnamed cartoon animal characters, spare layout, and droll messages...sold between 1973 and 1995. The best known of these is a 1975 birthday card bearing images of four animals and the message "Hippo Birdie Two Ewes," a pun playing on the phrase "Happy Birthday to You." The card has sold over ten million copies to date. As the greeting cards were signed simply "Boynton," many consumers assumed the creator to be a man. Boynton reports having been often asked if she was related to "the guy who does the cards". (To which she customarily responded, "Only marginally.")

Since the 1977 release of Hippos Go Berserk!, Boynton has published many children's books, as well as several illustrated humor books for the general market. Her books are most typically for very young children, offered in the laminated paperboard format known as board books. Nearly all of Boynton's books have been published by either Workman Publishing or Simon & Schuster. Four of her books have been New York Times best sellers: Chocolate: The Consuming Passion (1982); Yay, You! (2001); Consider Love (2002); and Philadelphia Chickens (2002), which reached the number one position on the list, and was on the list for nearly a year. Three Boynton books are on the Publishers Weekly All-Time Bestselling Children's Books list. More than 20 million copies of her books have been sold.

In 1996, Boynton began writing and producing songs...which she has described as "renegade children's music" — with composer Michael Ford; these songs have been released as albums (Rhinoceros Tap, Philadelphia Chickens, and Dog Train) and also published as book and audio disc sets. The tracks were recorded, under Boynton's direction and Ford's musical direction, by an eclectic roster of actors and musicians, including Blues Traveler, Meryl Streep, Alison Krauss, Steve Lawrence and Eydie Gorme, John Ondrasik of Five for Fighting, Kevin Kline, Laura Linney, "Weird Al" Yankovic duetting with Kate Winslet, Patti LuPone, The Bacon Brothers with Mickey Hart, Eric Stoltz, the Spin Doctors, Mark Lanegan, Hootie & the Blowfish, Natasha Richardson, Billy J. Kramer, Scott Bakula, Eric Bazilian and Rob Hyman, The Phenomenauts, and Davy Jones of The Monkees. Boynton received a 2003 Grammy nomination for Philadelphia Chickens. All three of these albums have been certified gold by the RIAA. Her fourth album, Blue Moo: 17 Jukebox Hits from Way Back Never, was released in November 2007, and includes tracks sung by Brian Wilson, Neil Sedaka, B.B. King, Sha Na Na, Bobby Vee, and Gerry & The Pacemakers.

She has written the text for four choral pieces composed by Fenno Heath, Director Emeritus of the Yale Glee Club, all of which have been performed by the Yale Alumni Chorus on international tour.

In 2008, Boynton ventured into filmmaking, creating and directing music videos of her most popular recorded songs. Her first music video, to be released in November 2009 as a book/DVD combination, is One Shoe Blues, starring B.B. King and a cast of assorted sock puppets. In development are Penguin Lament starring John Ondrasik of Five for Fighting, Philadelphia Chickens which is mostly animation and includes cameos by Kevin Bacon and Michael Bacon, and the fully-animated Cows.

The third of the four daughters of Jeanne (née Ragsdale) and Robert W. Boynton, Sandra was born in Orange, New Jersey, and grew up in the Mount Airy section of Philadelphia, Pennsylvania. Her father was a noted progressive educator, scholar (collaborating on textbooks with Shakespearean scholar Maynard Mack), and publisher and co-founder of Boynton/Cook Publishers, now owned by Heinemann.

Boynton's parents became Quakers when she was two years old. From kindergarten through 12th grade, she and her sisters attended Germantown Friends School, where their father taught English and was Head of the Upper School. Boynton has frequently cited as central to her own "upbeat offbeat" sensibility Germantown Friends’ arts-centered curriculum, as well as its thorough integration of the values of pacifism, independent inquiry, and individualism. She also spent part of her 10th grade year at Ackworth School near Pontefract, England.

She went on to Yale, entering in 1970 in the college's second year of coeducation. She spent the second semester of her junior year studying in Paris through Wesleyan University's program. At Yale, she majored in English, and also sang sporadically with the Yale Glee Club; she had joined the Glee Club when additional singers were needed for a performance of Beethoven's Ninth Symphony at Carnegie Hall, under the direction of Leopold Stokowski. Boynton has described herself as "an enthusiastic but undistinguished alto." At her graduation from Yale in 1974, she received a Special Master's Magna solemnly bestowed by Charles Davis, the Master of Boynton's residential college, Calhoun College. Unbeknownst to the graduation audience, the honor was actually a fiction. Boynton's grade point average did not in fact entitle her to any degree honor whatsoever; but shortly before the ceremony, she had told Professor Davis in mock earnest that "my parents are here, so I’d really appreciate it if you could just mumble some Latin after my name."

She studied Latin for five years in high school...not so much out of a scholarly passion for classics but rather as an avoidance of science classes, the scheduling of which invariably conflicted with Latin. During her undergraduate and graduate years, her teachers included Cleanth Brooks, Harold Bloom, Richard B. Sewell, Maynard Mack, Maurice Sendak, Richard Gilman, Rocco Landesman, David Milch, Stanley Kauffmann, and William Arrowsmith. In an autobiographical talk given at Yale in 2002, "The Curious Misuse of a Yale Education", Boynton refers to her book Grunt (an illuminated book and recording of plainchant in Latin and Pig Latin) as "the culmination of a lifetime spent joyfully squandering an expensive education on producing works of no apparent significance."

Boynton intended to become a theater director. For graduate studies in drama, she attended the University of California at Berkeley for one year, then transferred to the Yale School of Drama D.F.A. program, but she did not complete the program. With the birth of her first child in 1979, Boynton postponed indefinitely a career in the theater, judging the demands of that profession not easily compatible with raising a family. She has been slowly returning to directing work: in May 1995, she wrote and directed a benefit reading, On Stage...featuring Jill Clayburgh, Joe Pacheco, and Jane Curtin...for Sharon Stage in Connecticut; in November 2005, and again in November 2007 she presented songs from Philadelphia Chickens, Dog Train, and Blue Moo at the Kennedy Center's Millennium Stage; and in November 2006, she directed her son Keith in his own play, The Quotable Assassin, Off-Off-Broadway at Alternate Stages.

In 1978, Boynton married a fellow Yalie, the writer and Olympic athlete Jamie McEwan, bronze medalist in the 1972 Summer Olympics in the C-1 event. McEwan was also in the 1992 Olympic Games, placing fourth in doubles canoe; in 1991, Boynton and McEwan moved with their children to the Hautes-Pyrénées region of France for a year, so that McEwan and his doubles partner, Lecky Haller, could train with the French team. McEwan has been a member of several whitewater expeditions, to Mexico, Bhutan, British Columbia, and a National Geographic—sponsored descent of part of the Tsang-Po River (Brahmaputra) in Tibet, an ill-fated trip detailed in The Last River by Todd Balf, and in Courting the Diamond Sow by expedition leader Wickliffe W. Walker. Boynton has illustrated two of McEwan's five children's books. They have four children: Caitlin McEwan, an actress; Keith Boynton, a playwright; Devin McEwan, a whitewater racer and member of the 2001 U.S. Team; and Darcy Boynton, a student. All four children are singers as well, and each performed on the Philadelphia Chickens recording.

Boynton received the Irma Simonton Black Award for Chloe and Maude, the National Parenting Publications Gold Medal for Barnyard Dance and for Your Personal Penguin, a Grammy Award Nomination for Philadelphia Chickens, the Eustace D. Theodore Fellowship (Yale University), the National Cartoonists Society Greeting Card Award for 1992, and the National Cartoonists Society Book Illustration Award for Blue Moo: 17 Jukebox Hits From Way Back Never, in 2008. She is the 2008 recipient of the Milton Caniff Lifetime Achievement Award, the National Cartoonists Society's highest honor.

Children's books

• Hippos Go Berserk! (1977)
• Hester in the Wild (1977)
• If At First (1980)
• A to Z (1982)
• Blue Hat, Green Hat (1982)
• Doggies (1982)
• Horns to Toes (1982)
• The Going to Bed Book (1982)
• Moo, Baa, La La La! (1982)
• Opposites (1982)
• But Not the Hippopotamus (1982)
• A is for Angry (1983)
• The Story of Grump & Pout (written by Jamie McEwan) (1988)
• Birthday Monsters! (1993)
• Barnyard Dance! (1993)
• One, Two, Three! (1993)
• Oh My Oh My Oh Dinosaurs! (1993)
• Rhinoceros Tap and 14 Other Seriously Silly Songs (book and audio CD) (1996)
• Snoozers (1997)
• Dinosaur's Binkit (1998)
• BOB and 6 More Christmas Stories (1999)
• Dinos To Go (2000)
• Hey!, Wake Up! (2000)
• Pajama Time! (2000)
• The Heart of Cool (written by Jamie McEwan) (2001)
• Philadelphia Chickens (book and audio CD) (2002)
• Snuggle Puppy! (2003)
• Fuzzy Fuzzy Fuzzy (2003)
• Moo Cow Book (cloth) (2004)
• Belly Button Book! (2002)
• Dog Train: A Wild Ride on the Rock-and-Roll Side (book and audio CD) (2005)
• Your Personal Penguin (book with song download) (2006)
• What's Wrong, Little Pookie? (2007)
• Bath Time! (2007)
• Blue Moo: 17 Jukebox Hits from Way Back Never (book and audio CD) (2007)
• Fifteen Animals! (2008)
• Barnyard Bath! (2008)
• Let's Dance, Little Pookie (2008)
• Night-Night, Little Pookie (2009)
• One Shoe Blues (book and DVD) (2009)
• Happy Birthday, Little Pookie (2010)
• Perfect Piggies (2010)
• Are You a Cow? (customizable board book) (2010)

General market books

• Gopher Baroque (1979)
• The Compleat Turkey (1980)
• Chocolate: The Consuming Passion (1982)
• Don’t Let The Turkeys Get You Down (1986)
• Christmastime (1987)
• GRUNT Pigorian Chant (book and audio CD) (1996)
• Yay, You! (2001)
• Consider Love (2002)