Andrew Barrett, 32, from Norfolk, worked for the East of England Ambulance Service but resigned when he was charged.
The girl was aged 14 and 15 when she was sexually assaulted in summer 2013 and May last year.
Barrett, of Rayners Way, Mattishall, who was not on duty when he committed the offences, was jailed for 16 months.
Norwich Crown Court heard the father of one met the girl while doing voluntary work and began sending her sexually explicit messages on Whats App and by text.
He also texted the girl naked photographs of himself and became "persistent in his requests" for her to do the same, the court was told.
Martin Ivory, prosecuting, said she eventually sent him five pictures.
During the first sexual assault, he hugged her when she became upset and touched her indecently.
The second took place while he was driving and she was in the passenger seat. He again touched her indecently.
In a victim impact statement read to the court, the girl said she became anxious every time she heard an ambulance siren and at the prospect of ever having to call an ambulance.
She said she had nightmares and had self-harmed.
David Wilson, defending, said Barrett had been a man of "exemplary conduct" and had worked for the ambulance service for 13 years.
Disregarding Mr Wilson's call for a suspended sentence, Judge Antony Bate said: "I would be failing in my public duty if I didn't send you to prison immediately today."

The group says it has 1.5 terabytes of the company's data and has posted episodes of Ballers and Room 104 online.
It added that more material would be released "soon".
HBO confirmed it had experienced a "cyber incident" in a statement.
In an email published by Entertainment Weekly, the hackers appeared to offer more details in exchange for favourable coverage.
"Hi to all mankind," they wrote. "The greatest leak of cyber space era is happening."
They encouraged recipients to download the material and added: "Whoever spreads well, we will have an interview with him."
Reports have said the allegedly stolen Game of Thrones script appears to be from the fourth episode of season seven, which is currently being broadcast.
The BBC has not been able to independently verify that the hackers possess the material they claim to have stolen.
HBO confirmed that a "cyber incident" had resulted in the compromise of information.
"We immediately began investigating the incident and are working with law enforcement and outside cybersecurity firms," the firm added.
"Data protection is a top priority at HBO, and we take seriously our responsibility to protect the data we hold."
The intrusion was "obviously disruptive, unsettling, and disturbing for all of us," said chairman and chief executive Richard Plepler in an email to HBO employees.

The Friends of Sherwood Forest say the Â£5.3m centre, located on the edge of a country park, will devastate the area.
"It has all sorts of wildlife - insect life and newts," the charity's group spokesman Adrian Wilson said.
Nottinghamshire County Council said the new site would "balance the interests" of the community and visitors.
The current centre is in Sherwood Forest Country Park, on land designated as a site of special scientific interest but plans are to move it to Forest Corner, on grassland on the edge of the park.
The council has signed a contract with the Royal Society for the Protection of Birds (RSPB) to build and run a new one.
County council spokesman Derek Higton said: "The centre will be run by one of the world's leading conservation organisations - and that's why we partnered with them."
Updates on this story and more from Nottinghamshire
"We are confident we can balance the interests of the community and the interests of the 350,000 people who visit Sherwood Forest and Major Oak every year."
Detailed plans have not yet been submitted.
Mr Wilson said: "It is priceless - if you were to drive up the road, on a Sunday lunchtime, you would see the cricketers out here all in their whites, people sat round watching the cricket match.
"You get that feeling straight way - this is England."
Shawn and Linda Geery, who walk their dog in the country park, said: "It is too near the edge of the forest, the parking problems will be horrendous."
Construction is scheduled to start in December 2016 with the opening expected by winter 2017.
A paramedic who admitted sexually assaulting a teenager and possessing indecent images of her has been jailed.
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A group of hackers claims to have stolen the script for a forthcoming Game of Thrones episode and other data in a breach at entertainment firm HBO.
[NEXT_CONCEPT]
A new visitors' centre proposed for Sherwood Forest will ruin "a priceless" tract of land and threaten wildlife, opponents say.