The 24-year-old was under contract with the Stags, but Marcus Bignot's side have reached an agreement to bring him in on a free transfer.
He spent the first half of the season on loan at Crawley, scoring three goals in 23 appearances.
The ex-Burton player will officially join on 1 January and becomes Bignot's first signing for the Mariners.

Raqqa Is Being Slaughtered Silently (RBSS) said Ahmad Mohammed al-Mousa was killed by a group of masked men in the rebel-held city of Idlib on Wednesday.
The group have no further details, but at least two other members have been killed by IS militants since 2014.
RBSS was honoured last month by the Committee to Protect Journalists (CPJ).
The New York-based organisation condemned Mr Mousa's murder.
"Just a few weeks ago, nearly 900 journalists, press freedom advocates, and supporters attending CPJ's International Press Freedom Awards stood together in solidarity with Raqqa Is Being Slaughtered Silently. Today we are all standing together again, this time in mourning," said CPJ Executive Director Joel Simon.
Earlier this year, IS claimed responsibility for the kidnapping of Mr Mousa's father and four other Syrians accused of being members of RBSS.
In October, RBSS member Ibrahim Abdul Qader was killed by IS militants, along with fellow journalist Fares Hamadi, in south-eastern Turkey.
IS subsequently published a video showing one of the men with his throat cut, and warned: "You will not be safe from the knife of the Islamic State. Our hand will reach you wherever you are."
RBSS is one of the few independent sources of news left in the northern Syrian city of Raqqa, which IS has controlled since August 2013 and serves as the de facto capital of the "caliphate" whose creation the group proclaimed in June 2014.

Forster, who rejoined the club in July after 22 games on dual registration at Haven earlier in his career, replaces sacked James Coyle.
He began his career at Saints, winning the academy Grand Final there, but was limited to five senior appearances.
"This appointment will lift everybody," general manager Amanda Hewer said.
The ex-Salford forward takes charge after a season which saw Haven relegated from the Championship, despite winning three of the final five games in the Super 8s.
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Marcel Lazar, known online as "Guccifer", pleaded guilty in May on charges of aggravated identify theft and unauthorised access of a computer.
Lazar, who targeted former Secretary of State Colin Powell and the Bush family, was arrested on hacking charges in Romania in 2014 and given four years.
He was extradited to the US to face charges in March 2016.
The 44-year-old inadvertently uncovered Democratic presidential nominee Hillary Clinton's private email account in 2013 after hacking the account of her former political adviser, Sidney Blumenthal.
Two years later, it was revealed that Mrs Clinton exclusively used her personal account, which was connected to a private server at her New York residence, while serving as secretary of state.
Mrs Clinton's use of private email while at the State Department became the subject of an FBI investigation and has dogged her on the campaign trail.
Earlier this year Lazar claimed he had hacked into her private server, but the Clinton campaign has denied there was any such breach.
The FBI also has said there is no evidence to prove he had hacked her.
Lazar, a former taxi driver in Romania, is believed to have hacked into about 100 US email accounts between 2012 and 2014.
In June, a hacker who goes by the alias "Guccifer 2.0" claimed data breaches on the Democratic National Committee and the Democratic Congressional Campaign Committee.
US intelligence officials have said they believe Guccifer 2.0 is linked to Russian intelligence services, suggesting that the hack was state-sponsored.
There is no known connection between Lazar and Guccifer 2.0.
Grimsby have signed striker Adi Yussuf on an 18-month deal from fellow League Two side Mansfield.
[NEXT_CONCEPT]
Another member of a Syrian activist group that reports on the activities of the jihadist group Islamic State (IS) in the city of Raqqa has been murdered.
[NEXT_CONCEPT]
Whitehaven have appointed former St Helens prop Carl Forster as player-coach at the age of 24, the youngest in the professional game.
[NEXT_CONCEPT]
A Romanian hacker who targeted high-profile US politicians has been sentenced to 52 months in prison.