John Leahy resides in the Belvidere section of Lowell with his wife Joni and their three children Delane, Madeline and Jack. He is the son of Daniel and Gloria Leahy. John has four siblings: Dan, Mark, Kathleen and Lynda.

Councilor Leahy is a graduate of Bishop Guertin High School and UMass Lowell where he earned a bachelor of science degree in Business Management. Councilor Leahy has owned and operated Leahy Painting for more than 25 years.

Councilor Leahy served on the Lowell School Committee from January 2001 to January 2011. While on the School Committee Councilor Leahy served as Vice Chair for the 2006-2007 term under Mayor William F. Martin.

Councilor Leahy has served on many boards and civic organizations including Celebration of Life Jamaica and Lowell Mental Health. Councilor Leahy has coached for Lowell Youth Soccer, Shedd Park Baseball and Lowell Lacrosse.