The NYC native was raised in Yonkers, the son of a druggist father and high school teacher mother. As a college freshman, Masur began as a pre-med majoring in anthropology. Accompanying a friend to an audition for a school play, he was asked to read and won the part. Masur soon switched his major to Theater Arts, and later pursued his study of acting at the Yale School of Drama. He racked up stage experience as an actor and technical director at the Hartford (CT) Stage Company. In 1973, Masur originated a role in the New Haven production of the rugby drama "The Changing Room" before moving with the show to Broadway. TV writer-producer Norman Lear saw Masur in a performance and invited him to do a guest shot on "All in the Family". Masur made his TV acting debut on February 2, 1974 playing a mentally challenged delivery boy who becomes smitten with Gloria (Sally Struthers). He moved from NYC to Los Angeles six months later.
In the fall of 1974, Masur made a memorable guest shot on "The Mary Tyler Moore Show". By the following year, he was co-starring as the harried manager of "Hot L Baltimore" (ABC, 1975). This quickly led to work as a regular on the first season of "One Day at a Time" (CBS, 1975-76), and a recurring role on the "MTM" spin-off "Rhoda" (CBS, 1975-76). 1976 marked Masur's debut in TV-movies in "Having Babies", an ABC medical drama. Though a frequent series guest star and the star of several busted sitcom pilots, he would receive most of his best roles in "social problem"-oriented TV-movies.
Masur proved chilling as a producer of child pornography in the well-received "Fallen Angel" (CBS, 1981). He continued to gravitate toward serious subjects with a supporting role in the lauded child abduction docudrama "Adam" (NBC, 1983) which he reprised for the sequel "Adam: His Song Continues" (NBC, 1986). Masur garnered an Emmy nomination playing the defense attorney of battered wife-turned-murderess in "The Burning Bed" (NBC, 1984). As a change of pace, he proved comfortable battling supernatural evil with the rest of the ensemble in the miniseries "Stephen King's 'It'" (ABC, 1990) as he was in the far more ambitious award-winning "And the Band Played On" (HBO, 1993). Masur's more recent telefilm highlights include a supporting role in "My Brother's Keeper" (CBS, 1995), an AIDS drama, and in "Hiroshima" (Showtime, 1995).
Segueing to directing, Masur helmed an Oscar-nominated live-action theatrical short "Love Struck" (1986). He subsequently garnered a Directors Guild of America Award nomination for helming "Torn Between Two Fathers" (ABC, 1989), a presentation of "ABC Afternoon Specials". Masur's other directing credits include episodes of "The 'Slap' Maxwell Story", "The Wonder Years", and "Picket Fences" (a series on which he played the recurring role of reactionary postal worker-turned-mayor Ed Lawson).
Masur was elected third vice-president of the Screen Actors Guild in 1993 and served as president from 1995 to 1997.