Kelly is a long-time resident, having moved to Fort Collins in 1973 to attend graduate school at CSU. He taught public school science and math and renovated historic buildings in downtown Fort Collins. Kelly was first elected to City Council in 1983 and served as mayor in the mid- 1980s. He served three terms as Larimer County Public Trustee (appointed by the governor). He was elected to the City Council in 2005 and re-elected in 2009, serving three terms as Mayor Pro Tem during that time. He was elected again in April 2021.

Kelly was a leader of eight open space and natural areas measures that have protected over 75,000 acres in Fort Collins and Larimer County. He served eight years on the Natural Resources Advisory Board and also eight years on the Land Conservation Stewardship Board. Kelly has also served on the Poudre Fire Authority, Downtown Development Authority, Poudre Landmarks Foundation, the CDC Animal Care Committee, and many more boards and commissions. He has earned numerous awards and recognition for his leadership and public service, including for open space and natural areas protection, recycling, and historic preservation.

Kelly is married to Linda Stanley, a senior research scientist at CSU. Together they enjoy friends, good food, hiking, wildlife viewing, reading, movies, and garage sales.