Tejinesh Gebisa Tulu of Ethiopia won the women's portion of the race with a time of 2:30:16. Antonina Kwambai of Kenya was second at 2:30:19.
Richtman, who attended Montana State University, triumphed despite running in just his second marathon.
"It's definitely a surprise,'' he said moments after crossing the finish line. "You know I came in feeling really good; it's really hard to judge where you stack up against competition, but it was a really good group out there today, and I was really happy to run with those guys and then ultimately win it."
Richtman said he drew inspiration from the atmosphere in Los Angeles.
"It's such a great day for it, you know, the 40th anniversary, there's so many legacy runners out here, and the crowd out here was amazing. There were people throughout the whole course and it really helps. It's amazing how much a crowd can change the course of what you're doing.''
In an interview with ABC7's sister station WLS-TV, Richtman said: "I didn't really have a super big plan coming into this. It's always so tough, the marathon, just because so much can happen. I just told myself to stay in that pack until about the halfway mark and then if I felt good I could make a move. Ended up going a little bit to the lead and no one really came with me."
Athanas Kioko of Kenya was second for the men with a time of 2:10:55.
Illinois runner is 1st American man to win LA Marathon in 31 years
Moses Kiptoo of Kenya was third in 2:13:13.
Savannah Berry of Bakersfield finished third for the women with a time of 2:30:31.
"I'm beyond thrilled,'' Berry said. It was fun racing against this group of girls, and then honestly seeing Matt by me ... I was like, OK, yes, we got this. I wanted to represent for the state, and I wanted to represent for our country.''
Paul Pilkington was the last American to win the LA Marathon in 1994 in a time of 2:12:13. American Olga Appell won the women's race that year in a time of 2:28:12.
The capacity field for the 40th Los Angeles Marathon numbered more than 26,000.
The wheelchair racers took off from Dodger Stadium promptly at 6:30 a.m., followed by the hand crank racers at 6:35 a.m. and the women's elite field at 6:44 a.m.
The elite men's field hit the pavement at 7 a.m.
Several firefighters who fought January's devastating wildfires were among those participating in the 26-mile, 385-yard race.
U.S. Forest Service Capt. Jerry Garcia, who helped battle the Eaton Fire, helped co-pace the group of runners who hope to complete the marathon in two hours, 55 minutes.
The 47-year-old Palmdale resident was the top runner in the 45-49 age group in the 2024 Los Angeles Marathon with a personal best time of 2:40:13.
He won the 2017 Angeles Crest 100 trail running race.
Another firefighter in the field was Los Angeles Fire Department Firefighter Greg Balandran, who ran on behalf of the Widows, Orphans and Disabled Firefighters Fund, the official charity of the Los Angeles Firemen's Relief Association.
Shortly after the wildfires erupted, the fund began providing long-term support for the more than two dozen LAFD firefighters and their families who lost homes or were displaced.
The Los Angeles Marathon, New York Road Runners, the nonprofit organization that produces the New York City Marathon, the Chicago Marathon, and the Boston Athletic Association, organizers of the Boston Marathon, have come together to support Together LA, an initiative spearheaded by The McCourt Foundation, which owns and operates the Los Angeles Marathon, to aid those affected by the Los Angeles County wildfires.
The four organizations have co-branded limited-edition Together LA merchandise, with all net proceeds benefiting Community Organized Relief Effort and the California Fire Foundation. These funds will provide emergency relief, assist displaced families and support long-term recovery efforts.
Only the 2020 field of 27,150 entrants has had a larger field than Sunday's.
Dan Cruz, the head of communications for The McCourt Foundation, told City News Service the capacity field could be attributed to the marathon's growing reputation, "its scenic and iconic course, strong community support and a robust charity partner program.''
The field consisted of runners from all 50 states and more than 70 nations. It included 93 legacy runners who have run all 39 previous editions of the race while 40% are first-time marathoners.
Cruz said the large number of first-time marathoners "could be due to the explosion in popularity of social run clubs throughout the region.''
"We've also seen growth in the younger 20-29 year-old demographic and Students Run LA,'' the after-school mentoring and physical fitness program for at-risk middle and high school students, "has returned to pre-pandemic numbers with 3,500 participants,'' Cruz said.
Mayor Karen Bass was the honorary starter.
It was 49 degrees with winds of 4-5 mph at Dodger Stadium as the race began, National Weather Service meteorologist Andrew Rorke told City News Service.
Temperatures on the course will "warm up pretty quickly,'' rising to 60 by noon, Rorke said.
From Dodger Stadium, runners headed through Chinatown, downtown Los Angeles, Echo Park, Thai Town, Little Armenia, Hollywood, West Hollywood, Beverly Hills, Century City, Westwood and Brentwood then back through Westwood to Century City, with the finish line for the "Stadium to the Stars'' course on Santa Monica Boulevard, just past Century Park East.
The elite women started 16 minutes, 5 seconds ahead of the elite men for the Marathon Chase, with the overall first finisher receiving a $10,000 bonus.
The time difference was determined by projected winning times.
The chase was part of the marathon from 2004 to 2014, with women winning seven times and men four. It was discontinued in 2015 when the race served as the USA Marathon Championships. It was revived in 2022 with women winning each time since its revival.
The men's and women's winners will each receive $10,000, the second-place finishers $5,000 and third-place finishers $3,500. The men's and women's wheelchair winners will each receive $2,500.
The men's race has been won by a Kenyan 22 times since 1999, including 2024 when Dominic Ngeno won. Ethiopians won the other four times. A U.S. runner last won in 1994.
African women have won 12 of the last 15 races, including in 2024 when Kenyan Stacy Ndiwa won for the second consecutive year. Like with the men, a U.S. runner last won in 1994.
Francisco Sanclemente of Colombia won Sunday's wheelchair race with a time of 1:38:10, his third straight LA Marathon triumph. American Miguel Vergara was second in 1:43:32.
City News Service and WLS-TV contributed to this report.