McCaslin's father was a vibraphonist, and he played in his father's ensemble at the age of twelve. He had his own group in Aptos high school which played three years at the Monterey Jazz Festival. He studied under Paul Contos and Brad Hecht, both of whom played in his father's group. He played in a number of youth ensembles, most notably under the legendary band director and music educator Don Keller and toured Europe and Japan with them; he was offered a full scholarship to the Berklee College of Music, where he enrolled in 1984. While there he was influenced by Gary Burton, Herb Pomeroy, Billy Pierce, George Garzone, and Joe Viola. McCaslin performed regularly in Boston and Cambridge with The True Colors Big Band, run by composer Ken Schaphorst. In 1987 he joined Burton's group and toured the world with him for four years.
In 1991 he moved to New York City and was a member of Steps Ahead until 1994, replacing Michael Brecker. He also played with the Gil Evans Orchestra, the George Gruntz Concert Jazz Band, Danilo Perez, Maria Schneider, and Santi DeBriano. In 1996 he played on Ken Schaphorst's Uprising, alongside John Medeski, Doug Yates, and Uri Caine. The following year he played with David Binney, Scott Colley, and Kenny Wolleson on the collaboration Lan Xang. In 2006 he joined the Dave Douglas Quintet. In 2010 he recorded as a featured soloist on the CD Coming Through Slaughter - The Bolden Legend. His first record as a leader appeared in 1998, and he has continued issuing releases under his own name into the late 2000s.