Kerwin grew up in the Chicago suburb of Flossmoor, Illinois. After graduating from USC's Film School in 1972, he moved to New York City to pursue acting on the stage. After several off-off Broadway productions, he was cast for a brief stint on "The Young and the Restless", in Los Angeles. He ended up staying in LA for the next 15 years, working steadily in films, TV and on stage.

He worked regularly in TV, from standard network fare to recurring roles in Roseanne (1988), The West Wing (1999), and HBO's The Knick (2014). He appeared in numerous TV movies and miniseries and starred in Showtime's critically acclaimed series, Beggars and Choosers (1999).

In feature films Kerwin landed the role of the errant ex of Sally Field in Murphy's Romance (1985), and he played opposite Harvey Fierstein in the groundbreaking film, Torch Song Trilogy (1988) (a role he also played in the Tony Award winning play). He starred with Robin Williams in Jack (1996), Michelle Pfeiffer in Love Field (1992), and Sissy Spacek in Hard Promises (1991). He also appeared in 27 Dresses (2008) and played the father of Emma Stone in The Help (2016).

The focus of Kerwin's work has always been the stage. He has performed in regional theatres throughout the country, including South Coast Rep, San Diego's Old Globe, The Seattle Rep and The Mark Taper Forum in Los Angeles. He and his wife, Jeanne, a theatrical producer, moved to Manhattan's Upper West Side in 1993, where they raised their three children. While there, he worked extensively both on and off Broadway, culminating with his 19 month run in Tracy Lett's Tony and Pulizer Award winning play, August: Osage County.

Widowed in 2016, Kerwin resides in NYC.