Thomas DeVany Forrestall was born in Middleton, Nova Scotia, on March 11, 1936, one of four children in a Roman Catholic family. At age seven, he developed epilepsy and began having seizures, a condition that would affect him throughout his life. From 1942 to 1951, his family lived in Dartmouth, Nova Scotia, where his father, a carpenter, worked on the construction of a new hospital. During this period, Forrestall participated in Saturday morning art classes at the Nova Scotia College of Art in Halifax. He continued drawing and painting during high school in Middleton, where his early interest in art was stimulated by books on the subject.

In 1954, Forrestall was awarded a scholarship to study art at Mount Allison University in Sackville, New Brunswick. His studio teachers included Lawren P. Harris, head of the Fine Arts department, Ted Pulford, who taught watercolour painting, and Alex Colville. Colville introduced him to egg tempera, the medium in which Forrestall would specialize from the 1960s onward, and described him as "the most promising artist our school has produced in a decade." After graduating with his Bachelor of Fine Arts degree in 1958, Forrestall received a Canada Council grant that allowed him to travel and study in Europe. He married fellow student Natalie LeBlanc of Atholville, New Brunswick.

Upon returning to Canada in 1959, Forrestall and his wife settled in Fredericton, New Brunswick, where he was hired as assistant curator of the newly opened Beaverbrook Art Gallery. He was responsible for cataloguing the museum's holdings and maintaining accession records. His employment was terminated in early 1960 after he suffered two epileptic seizures at work. He subsequently worked as an editorial cartoonist for the Fredericton Daily Gleaner and as a designer for UNB Press, and taught art classes one night a week in a converted bakery building that patron Brigadier Michael Wardell made it possible for him to purchase. With income from these various activities, he was able to devote increasing time to painting and became a full-time professional artist in 1960.

Forrestall worked primarily in egg tempera and watercolour, developing a distinctive approach within the realist tradition often described as "Magic Realism." By 1963, he began experimenting with panels in shapes other than the traditional rectangle, a practice that became characteristic of his work. In 1965, he held his first solo exhibitions at Roberts Gallery in Toronto. In 1960, the New Brunswick government commissioned a painting by him as a wedding present for Princess Margaret. He was one of six artists commissioned to provide murals for the Centennial Building in Fredericton, which opened in 1967; his contribution was a sheet metal construction depicting farming. In 1972, he created the distinctive mural for the fly tower added to the Fredericton Playhouse, making it one of the most recognizable buildings in Atlantic Canada.

Forrestall was elected to the Royal Canadian Academy of Arts in 1973. In 1974, he collaborated with poet Alden Nowlan on "Shaped by This Land," juxtaposing 54 poems with 76 paintings and sketches. In 1986, his portrait of Pierre Trudeau's three sons, commissioned by the Canadian government, was presented to the former prime minister as a gift from the nation. In 2006, Forrestall appeared as a witness in an ownership dispute between the Beaverbrook UK Foundation and the Beaverbrook Art Gallery, testifying that he understood from his work as assistant curator and his contact with Lord Beaverbrook that valuable paintings in question were gifts to the gallery. His testimony was important in the case, which was decided in the gallery's favor.

A major retrospective of Forrestall's work at the Art Gallery of Nova Scotia in 2008 included work from the 1950s to 2007 and subsequently traveled to the Beaverbrook Art Gallery, the Confederation Centre Art Gallery, the McMichael Canadian Art Collection, and the Owens Art Gallery at Mount Allison University. Throughout his career, Forrestall maintained over 400 sketchbooks spanning seventy years, containing drawings, small wash paintings, and writings on his art and life. He donated 270 of these sketchbooks to the Beaverbrook Art Gallery, with a further collection of 100 sketchbooks exhibited at St. Thomas University in Fredericton in 2022 and 2023. His works are held in major collections including the National Gallery of Canada, the Art Gallery of Nova Scotia, the Musée d'art contemporain de Montréal, and the Art Gallery of Ontario. Tom Forrestall died in Dartmouth, Nova Scotia, on November 15, 2024, at age 88.