Born Lynn Ridgway in Collingwood, Ontario, she was raised in North Vancouver, British Columbia. She attended the Vancouver School of Art with hopes of making a living as an artist. After working briefly in animation, she married in 1969, and moved back to Ontario, where she worked as a medical artist at McMaster University for five years. Johnston's illustrations are currently in storage in McMaster's medical archive. They include depictions of routine hospital happenings, such as a father smoking in the waiting room.

While expecting her first child, she drew single-panel cartoons for the ceiling of her obstetrician's office. Those drawings were published in her first book, entitled David We're Pregnant, which was published in 1973. After her divorce, she did free-lance commercial and medical art in a greenhouse which was converted into a studio. Hi Mom! Hi Dad!, a sequel to David, was published in 1975. Shortly thereafter, she met and married dental student Rod Johnston.

In 1978, the Johnstons and their two children relocated to Lynn Lake, Manitoba. She was asked by Universal Press Syndicate if she was interested in doing a comic strip. She sent off twenty copies of a strip called The Johnstons, based on her own family "since we were the only people I knew I could draw over and over again with some consistency." Much to her surprise, the syndicate approved of the initial strips and offered her a twenty-year contract. After a six-month "work-up" period, the strip first appeared in newspapers throughout Canada under the title For Better or For Worse. The strip is currently carried by about 2000 newspapers in Canada, the U.S. and 20 other countries.

Many story lines draw from her family's real-life experiences. Her main characters are named after the middle names of her husband and children. Elly is named after a friend who died when Johnston was young. Her brother-in-law Ralph Johnston inspired the controversial story about Lawrence's coming out. Deanna was based on Johnston's son's high school sweetheart, who died in a car accident years after her relationship with Johnston's son ended. Johnston's niece Stephanie is developmentally handicapped and her experience is shared in recent story lines on the integration of developmentally handicapped students in April's class.

The characters in For Better of For Worse have aged in "real time". On August 31, 2008, Johnston herself appeared in the Sunday strip, which was supposed to be the end of the cartoon, and announced that she would take the story back nearly 30 years to soon after its beginning, with half of the material to be new and the other half repeats.

Since the 1990s, Johnston has been notably forthcoming in her discussion of the abuse inflicted on her by her mother, her first husband, and being unprepared to be a mother to her son Aaron...topics which have also been reflected in the strip. A column by Jan Wong of the Globe & Mail, reprinted in Lunch With Jan Wong disclosed Johnston's difficult personality and considerable irascibility, and provided an alternate insight into her some of her complaints of being victimised by persons in her life.

Johnston now resides in the Northern Ontario town of Corbeil. Her daughter Katie lives in Corbeil and works at the For Better or For Worse studio, while her son Aaron works in the television industry in Vancouver, BC. In September 2007, Lynn and Rod Johnston announced their separation and intention to divorce. Johnston had talked about either ending For Better or For Worse or handing it off to another cartoonist, but changed her mind as a result of her split from her husband of over 30 years.

Johnston claimed a close friendship with Charles M. Schulz, creator of Peanuts.

• 1985 - Reuben Award, the first woman and first Canadian to win
• 1987 - Gemini Award, Best Cartoon Series
• 1990 - Honorary Degree, Doctor of Letters, Lakehead University, Thunder Bay, Ontario
• 1991 - National Cartoonist Society Newspaper Comic Strip Award
• 1992 - Made a Member of the Order of Canada, the country's highest civilian honour
• 1993 - Honorary Degree, Doctor of Laws, McMaster University, Hamilton, Ontario
• 1993 - Nominated for the Pulitzer Prize for a story on Lawrence's coming out
• 1999 - Honorary Degree, Doctor of Laws, University of Western Ontario, London, Ontario
• 2000 - Honorary Degree, Doctor of Letters, Nipissing University, North Bay, Ontario
• 2000 - Honorary Degree, Doctor of Letters, Emily Carr Institute of Art and Design, Vancouver
• 2001 - Comic of the Year, Editor and Publisher
• 2003 - A star on Canada's Walk of Fame in Toronto
• 2004 - Debwewin Citation from the Anishinabek Nation for excellence in Aboriginal-issues journalism
• 2007 - the Order of Manitoba
• 2008 - Inducted into the Canadian Cartoonist Hall of Fame

• Date unknown - Inducted into the National Cartoon Museum Hall of Fame