On January 27, 2011, Onalfo was named assistant coach for the Los Angeles Galaxy, reuniting with head coach Bruce Arena, with whom he formerly coached for the U.S. National Team and played for at D.C. United in 1998. He was also named head coach of the Galaxy's reserve team the same day. Onalfo continued serving as reserve coach when the reserve team entered the United Soccer League as a full club. Following the departure of Arena to coach the United States national team, Onalfo was promoted to first team head coach for the 2017 season. However, once again, this tenure lasted less than a full season, with Onalfo being fired on July 27, 2017 after a 6-10-4 record.