In her career as a librarian, she worked in many locations including librarian for the elementary schools of the Central Hawaii School district and as cataloguer in the Hilo Library in Hawaii; she supervised the compilation of the Union Catalog of Art in Chicago; and was a research librarian with International Harvester in Chicago, Illinois. She worked in Florida as a librarian at the Naval Operating Base in Key West, Florida, and organized and administered the library at the Naval Air Technical Training Center. She did research in children’s literature for Scott, Foresman, and Company; served as the serials librarian of the Chicago Undergraduate Division of the University of Illinois; and was an editor of the Illinois Women’s Press Association’s monthly bulletin, Pen Points.

MacGregor did not begin to write for publication until 1946. A story that she submitted at that time for a class at the Midwestern Writers Conference was later published as the book Tommy and the Telephone. The first appearance of Miss Pickerell, her famous and quirky major character was in a short story, "Swept her into Space," published in Liberty in 1950. This short story was later expanded to book-length in 1951's Miss Pickerell Goes to Mars, a science fiction novel for children. MacGregor's goal was to provide fantasy literature with correct (for the time) scientific facts that would appeal to children Miss Pickerel was well-received by critics, such as Virginia Kirkus, who wrote: "A lively novelty. Fact and fancy in a new venture for this age group." It was also released as the initial selection of the new Weekly Reader Children’s Book Club. Three other Miss Pickerel adventures followed before MacGregor's early death.

Besides these books, many of MacGregor's stories appeared in magazines of that period, such as Story World, The Instructor, and Christian Home.

Books Written by MacGregor Herself

McGraw Hill published one Miss Pickerel and two independent children's books written by MacGregor after the author's death.

• Tommy and the Telephone. illustrated by Zabeth. Chicago. Whitman. 1947.
• Miss Pickerell Goes to Mars. illustrated by Paul Galdone. New York. McGraw Hill, 1951: London. Blackie. 1957.
• Miss Pickerel! and the Geiger Counter. illustrated by Paul Galdone. New York. McGraw Hill. 1953: London. Blackie. 1958.
• Miss Pickerell Goes Undersea. illustrated by Paul Galdone. New York. McGraw Hill. 1953: London. Blackie. 1959.
• Miss Pickerell Goes to the Arctic. illustrated by Paul Galdone. New York. McGraw Hill. 1954: London. Blackie. 1960.
• Theodore Turtle. illustrated by Paul Galdone. New York. McGraw Hill. 1955: London. Faber. 1956.
• Mr. Ferguson of the Fire Department. illustrated by Paul Galdone. New York. McGraw Hill. 1956.
• Mr. Pringle and Mr. Buttonhouse. illustrated by Paul Galdone. New York. McGraw Hill. 1957.

Books Completed by Dora Pantel from MacGregor's Notes

Following Ellen MacGregor's death, McGraw-Hill searched unsuccessfully for someone to continue the series until 1964, when they selected Dora Pantell, a social worker, and a writer of publicity and educational curricula. In addition to children's books, Pantell wrote numerous textbooks and manuals, most dealing with the teaching of English as a second languageThe Miss Pickerel Series.. Since the later books of the series deal with issues or themes that did not exist or were not apparent during MacGregor's lifetime (energy crisis, computers, artificial satellites), it seems clear that Ms. Pantell was continuing "in the spirit" of Ellen MacGregor and Miss Pickerel.

• Miss Pickerell on the Moon. with Dora Pantell. illustrated by Charles Geer. New York. McGraw Hill. 1965
• Miss Pickerell Goes on a Dig. with Dora Pantell. illustrated by Charles Geer. New York. McGraw Hill. 1966
• Miss Pickerell Harvests the Sea, with Dora Pantell. illustrated by Charles Geer. New York. McGraw Hill. 1968.
• Miss Pickerell and the Weather Satellite. with Dora Pantell, illustrated by Charles Geer. New York. McGraw Hill. 1971.
• Miss Pickerell Meets Mr. H.U.M. with Dora Pantell. illustrated by Charles Geer. New York. McGraw Hill. 1971.
• Miss Pickerell Takes the Bull by the Horns, with Dora Pantell. illustrated by Charles Geer. New York. McGraw Hill. 1976.
• Miss Pickerell to the Earthquake Rescue. with Dora Pantel. illustrated by Charles Geer. New York. McGraw Hill. 1977.
• Miss Pickerell and the Supertanker. with Dora Pantel. illustrated by Charles Geer. New York. McGraw Hill. 1978.
• Miss Pickerell Tackles the Energy Crisis. with Dora Pantel. illustrated by Charles Geer. New York. McGraw Hill. 1980.
• Miss Pickerell on the trail. with Dora Pantel. illustrated by Charles Geer. New York. McGraw Hill. 1982.
• Miss Pickerell and the Blue Whales. with Dora Pantel. illustrated by Charles Geer. New York. McGraw Hill. 1983.
• Miss Pickerell and the war of the computers. with Dora Pantel. illustrated by Charles Geer. New York. F. Watts. 1984.
• Miss Pickerell and the Lost World. with Dora Pantel. illustrated by Charles Geer. New York. F. Watts. 1986.

The Miss Pickerell Series

MacGregor is best known as an author from the Miss Pickerel children's science fiction series, although only four of them were published in her lifetime. She left sufficient notes on planned future novels to enable Dora Pantel to write and publish Miss Pickerel books, the last one, Miss Pickerell and the Lost World, appearing in 1986.

Miss Lavinia Pickerell is an unlikely heroine: prim, spinsterly, angular and stiff, wearing old?fashioned clothes and an outlandish hat, and devoted to her pet cow, she nevertheless manages to inadvertently stowaway on a rocket to Mars in her first adventure. But with her common-sense, practicality, and unflappable demeanor she manages to bring each adventure to a satisfactory conclusion. She is every child's favorite maiden aunt, and is possibly modeled after MacGregor herself.

The series resembles others which, long after their original authors' deaths (e.g. Tom Swift and Nancy Drew) continue under the pens of successor authors. Only one other author has written in the Miss Pickerel "universe"The Miss Pickerel Series. Iowa Public Schools. Access 21 June 2008., Dora Pantel, who was also responsible for bringing other Ellen MacGregor-conceived projects to completion.

Later writers who indicated that Miss Pickerel had been either an influence or a favorite include such authors as Harry TurtledoveAuthor/Editor Interview at Baen Books. “I think I found the Miss Pickerel stories and the Mushroom Planet books when I was in about the third grade." Accessed 21 June 2008., Susan Page DavisAt the Chat 'n' Chew Cafe' with Crystal Laine Miller -- Interview with Susan Page Davis. Her Favorite Childhood Book: Miss Pickerel Goes to Mars. Accessed 21 June 2008.Susan Page Davis - Author. Accessed 21 June 2008., and Sam RiddleburgerSci Fi For Kids. With a "ridiculousness rating" of 3. Accessed 21 June 2008..

The Science in Miss Pickerel

MacGregor included valid scientific facts in her Miss Pickerel books. Some of the topics she addressed were weightlessness in space travel, atomic energy and carbon-14 dating, nuclear-powered submarines and the continental shelf, the "bends” affecting divers who surface too rapidly, and many others.

To Mars, to the Arctic, above the ground or below, Miss Pickerell is dauntless, intrepid, and eager. It's all good fun, but with a bonus. The scientific basis of each of the Miss Pickerell stories is scrupulously accurate. Although she was careful not to overburden her fragile plots with didactic passages explaining gravity, radiation, etc. Ellen MacGregor managed very skillfully to incorporate a good deal of information that a child reader could absorb almost without realizing it. With rare judgement, she gauged just how much to present to the 8-12’s who are Miss Pickerell’s audience. A clear picture in bald outline, rather than a mass of confusing and discouraging detail, is most apt to appeal to and instruct that active age group.Good science fiction, like good nonsense or fantasy. must be firmly grounded in the world of reality. RootsWeb at Ancestry.com, bio of Ellen MacGregor. Accessed 21 June 2008