Mendoza ran all 26.2 miles in 30 pounds of tactical gear.
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Nearly 50,000 runners from around the world and across the country took over city streets on Sunday for the Bank of America Chicago Marathon.
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Dean Tatooles, a Chicago attorney and adventure photographer, was taking pictures of a friend who was running the marathon when he spotted Mendoza.
"This is weird, there's a police officer in the middle of the runners," Tatooles said. "And then I saw he was wearing a bib. And then I saw he was wearing all the tactical equipment."
Tatooles then quickly pulled up his camera to capture the powerful moment, and even gave Mendoza some supporting words.
"This guy is a beast," Tatooles said. "And I ran up next to him and gave him a pat on the back and started screaming at him, and he looked at me. And apparently around the corner he was running pretty good, so maybe it worked out."
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The Chicago Police Department said Mendoza served in the Navy and is an avid runner. He completed the marathon to support the Chicago Police Memorial Foundation, which financially supports the families of fallen officers.
Mendoza also ran to highlight Chicago's first responders.
"While everybody else has the lightest shorts and shoes and everything, Jim's out there doing it with his full uniform on," said Phil Cline, Executive Director of the Chicago Police Memorial Foundation.
Cline said Mendoza and 80 others who ran the marathon helped raise over $160,000 for the nonprofit, making the medal earned at the end of the race truly priceless.
"You can't go backwards, you've got to keep moving forward," Cline said. "He showed us that not only can you do that on the street as a policeman, but he did it as an athlete."