A prolific and versatile character actor, Walsh, the son of a US Army controller, was born in San Francisco and raised in Europe. The family eventually settled in Newport, RI. He graduated from the University of Rhode Island and worked variously as a social worker, journalist, teacher and restaurant manager before deciding at age 30 to try his luck as an actor in New York. A decade of stage work followed. Walsh, whose birth name is James Patrick (he adopted 'J T' due to a typographical error), made his first appearance in the Manhattan Theater Club's production of "Yucca Flats" (1973) and went on to co-star in David Mamet's "American Buffalo" (1976), "Richard III" (1979), with Al Pacino, "Rose" (1981), with Glenda Jackson and Jessica Tandy, and Mamet's Pulitzer Prize-winning "Glengarry Glen Ross" (1984).
Since making his film debut in a bit part in "Eddie Macon's Run" (1983), Walsh generally played calculating executives, law enforcement officials and white collar workers. His memorable screen credits include two 1987 Barry Levinson films, "Tin Men", as Danny DeVito's boss, and "Good Morning, Vietnam", as Robin Williams' antagonist, and "Wired" (1989), as journalist Bob Woodward. He was featured in two of Mamet's feature directorial efforts, "House of Games" (1987), as a businessman, and "Things Change" (1988), as a hotel manager. Aficionados of art-house films will recall him as Annette Bening's mentor in Stephen Frears' "The Grifters" (1990), while others may remember him as a would-be defense witness who is silenced by his own code of honor in Rob Reiner's "A Few Good Men" or as Frank Fitzsimmons, the Teamster president who succeeded "Hoffa" (both 1992). Walsh is perhaps best remembered for his turn as Danforth 'Buster' Keeton, a yacht salesman who makes a pact with the devil in the Stephen King adaptation "Needful Things" (1993). He continued to provide insightful performances in roles as varied as a cynical legal adviser in John Dahl's "The Last Seduction", the attorney prosecuting Santa Claus in the remake of "Miracle on 34th Street" and the head of the Memphis FBI in Joel Schumacher's "The Client" (all 1994), as well as delivering a warning about the contagion in "Outbreak" and portraying Watergate co-conspirator John Erlichman in Oliver Stone's "Nixon" (both 1995) and appearing as a Senator advising the President on a hostage situation in "Executive Decision" (1996). He also shone delivering a monologue to fellow inmate Karl Childress (Billy Bob Thornton) in "Sling Blade" (also 1996) and was suitably menacing as a trucker in the surprise hit "Breakdown" (1997). Just before his untimely death in 1998, Walsh had completed roles in "Pleasantville" and "The Negotiator".
Walsh made his TV series debut as a shady Naval officer on the sci-fi drama "Dark Skies" (NBC, 1996-97).