Callahan attended and ran at North Shore Country Day School, where he was coached by Patrick McHugh.[2] By the time he graduated in the summer of 2009, he recorded personal bests of 1:51.22 in the 800 meters and 4:05.20 in the mile.[3]
Callahan first attended and ran at Princeton University, where he graduated in 2013. While at Princeton, he was coached by Steve Dolan and Jason Vigilante and set the school record in the indoor 1,000 meters at 2:20.78.[7] He first broke four minutes for the mile in the winter of his Junior year at Princeton with a time of 3:58.86 at the Sykes-Sabock Invitational at Penn State University.[8]
Due to skipping two seasons from multiple injuries, he got fifth year NCAA eligibility when he enrolled in a graduate program at University of New Mexico.
Callahan made an appearance as a professional rabbit in the 2015 Diamond League men's 3000 meters.[11] In May 2016, the IAAF approved Callahan to represent Belgium in international competition.[12] He was eligible to represent Belgium due to his mother having been born in Antwerp.[12]
The approval happened amidst a race campaign in hopes of qualifying for the 2016 Summer Olympics. His first international competition after his IAAF-mandated switch was the 2016 European Athletics Championships, where he raced in the men's 1500 meters in a time of 3:41.75. He did not progress into the finals. He did not qualify for the Olympics in 2016, having not satisfied the 3:36.19 Olympic "A" standard for the men's 1500 meters. His closest result was at the 2016 Swarthmore Final Qualifier, where he ran the 1500 in 3:37.87, placing seventh.[13] On July 22, 2016, Callahan raced in the men's mile at the 2016 Morton Games in Dublin, Ireland, where he finished in 3:56.14 for fourth place.[14]
