During Fairfield, Longwell played for Slough Town F.C. in 1978 in the English Isthmian League. After Fairfield, Longwell had multiple tryouts at the professional level. He then signed with the Tampa Bay Rowdiesof the North American Soccer League. He made his professional debut on August 5, 1983 against the Chicago Sting, in a 4–3 win. He also played for the 1983–84 Tampa Bay Rowdies Indoor team. On April 3, 1984, Longwell signed with the Fort Lauderdale Sun of the newly established United Soccer League. The Sun went on to win the 1984 USL championship with Longwell as captain.
After professional soccer, Longwell turned to running and triathlons, competing in over 300 races, including 56 marathons/ultramarathons. He has a PR of 3:13 for the marathon, 4:29 for 50K, 8:06 for 50 miles and 22 hours, 6 minutes for 100 miles in the Vermont 100 Mile Endurance Run in 1996. Longwell also has a personal best in the Ironman triathlon of 11:36 in the 1993 Ironman Canada race in Penticton, British Columbia, Canada. Along the way, Longwell raced in marathons/ultras in 9 different countries. In 2004, he competed in the Race Across Americabike race in the two man division with John Delia of Middletown, CT. The team crossed the USA in 8 days, 15 hours and 42 minutes to finish in third place in their division. He completed his 100,000th documented lifetime cycling mile on Thanksgiving Day, 2012. Additionally, Mark has averaged 13,000 miles per year since 2012 and rode 16,025 miles in 2015- his highest annual total to date. As of March, 2018, Longwell has ridden over 173,000 miles lifetime. In June 2018, Longwell competed in the Race Across the West, a 925 solo non stop race from Oceanside CA to Durango CO. Unfortunately, due to injuries, he withdrew at mile 405. On October 27, 2018 Mark participated in the World 6,12 and 24 hour time trial championships in Borrego Springs, CA. Mark finished 6th out of 13 in his age group in the 12 hour division with 212.4 miles covered. On November 2, 2019, Mark participated again in the World 6,12 and 24 hour time trial championships in Borrego Springs, CA. Total mileage this time was 222.0 - a 9.6 mile increase over 2018- and this was good for 4th place in the 50-59 agre group. Mark completed his 200,000th lifetime mile on the bike in November 2019.