Eisen Teo Guan Kuan, 29, who had worked as a reporter for The Straits Times, twice assaulted the victim who was 15 at the time of the offences.
Teo pleaded guilty of engaging in oral sex with the victim in a male toilet cubicle at a local park in early 2012.
He also admitted to having sex with the girl at his flat on June 23 that year when his wife was baking in the kitchen. He had lured the victim to come over to his place to help him paint his apartment, Channel News Asia reported.
Teo mentored about a dozen student journalists at the "IN Crowd" media club, which the victim was part of.
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Media reports said Teo took a special interest in the girl. He had read on the girl's blog that she was depressed.
The former journalist and the girl got romantically involved in the fourth quarter of 2011.
District Judge Hamidah Ibrahim told Teo he had been "audacious enough" to have done so while his wife was "metres away."
During sentencing, Judge Hamidah said Teo had clearly taken advantage of a young and immature person.
In mitigation, Teo's lawyers said he was remorseful and that he was not a sexual predator.