 Kelly played with the Flyers until the 1979–80 NHL season when he was traded to the Washington Capitals.He had a career year offensively in his first season with the Capitals, scoring 26 goals and 36 assists for a total of 62 points.After playing on a checking line with the Flyers, he was promoted to the Capitals' top line, alongside Jean Pronovost and Mike Gartner and proved to be an excellent complement to the two highly-skilled players, using his size to create space for them.