Captain Tobias - as he is called by his supporters - has cerebral palsy and cannot walk or stand unaided but completed a marathon by walking hundreds of metres a day using his walker, inspired by NHS fundraiser Captain Sir Tom Moore.
But, after completing that challenge, he started again - this time using a race runner for support.
Tobias was joined by hundreds of cheering supporters on Saturday near his Sheffield home as he completed his last leg with the 2012 Olympic heptathlon champion running beside him.
He has now raised more than £137,000 for Sheffield Children's Hospital and Paces School, where he attends in the city.
"It feels amazing to have completed both marathons," Tobias said at the end, "I feel fantastic."
"Thank you for clapping and cheering me every step of the way. You are all awesome".
Asked about beating Dame Jess to the finish line, he said: "I have a new catchphrase that describes that - it was a very great, super-duper joy.
"I'm so pleased that she came along. It's been fantastic meeting her."
Tobias said he was now taking a rest, but added: "I love doing challenges so who knows what will be around the next corner?"
Dame Jess said she had been following Captain Tobias's exploits.
"As a Sheffielder and a patron of the children's hospital, just to see what he's achieved so far is incredible," she said.
"And then he sent me a message on Twitter saying 'would I like to join him today and have a bit of a race with him?' - but he wouldn't give me a head-start.
"So I had to take him up on that challenge."
Sporting a Captain Tobias "Chuffed to Bits" t-shirt, Dame Jess said: "I've very proud to be on his side for this last little leg of his two marathons, which is incredible.
"The whole community has come out for him. It's raised the morale and mood of everyone and inspired so many people in so many different ways."
Tobias's mother Ruth Garbutt said the challenges, which started as a way of keeping him active during lockdown, have transformed her son.
"Massive thanks to Jess for coming today. It's meant the world to Tobias for Jess to be here. To run against an Olympic champion is pretty awesome," she said.
After his first challenge put his fundraising total past the £100,000 mark, Captain Sir Tom Moore sent Tobias his congratulations and he also received a message from Prime Minister Boris Johnson.
Tobias began his latest challenge - dubbed his "Tobiathlon" - on June 14 using a three-wheeled race runner supplied by the charity CP Teens and has completed around 1.2km each day.
The youngster, who also has autism, originally aimed to raise £500 when he began his fundraising efforts. Donations can be made to justgiving.com/crowdfunding/ruth-garbutt-3