Hearn signed a three-year contract with Grimsby Town on 17 June 2011, for a £30,000 transfer fee.
His first goal for Grimsby came 11 games into the 2011–12 season against Wrexham, a through ball from Craig Disley firing a low shot passed Chris Maxwell.
The following week he scored 4 goals against his former club Alfreton on 1 October 2011, scoring the equaliser on 57 minutes finishing from close range.
His second came 3 minutes later when Duffy fed Hearn a  ball over the top to shoot past the keeper, the hat-trick goal came in the 70th minute, firing a shot the keeper could only assist into the net.
Hearn's fourth goal of the afternoon came in the 86th minute when he took the ball past the defence and the keeper to slide home and put Grimsby 4–1 up, the game finished a 5–2 victory to the Mariners.
Two games later on 11 October 2011, Hearn scored his 2nd hat-trick of the campaign in a 5–2 win over Barrow, he scored the first after racing clear of the defence to shoot underneath the keeper, after Barrow equalised, Hearn scored his second in first-half stoppage time, a one-two with Craig Disley put him in on goal holding off his marker, he side-footed home this time with his left-foot, his third came on 75 minutes, muscling his way through to turn in the box and beat Hurst with a low shot from six yards.

Hearn helped Grimsby to a 5–0 rout over Ashington in the fourth qualifying round of the FA Cup on 29 October 2011, Coulson and Duffy combined on 38 minutes to put Hearn through, who skipped round Dryden the keeper, before his first shot effort was well saved by Dryden, but he made no mistake at the second attempt; two minutes later another ball from Robert Duffy to Hearn who then muscled through the defence, scoring past Dryden from 10 yards.

On 26 November 2011, he scored two more goals in a 2–1 victory at Kettering, scoring the first on 14 minutes when Hearn read a lofted ball forward and muscled past defender Jamie Navarro before slotting past goalkeeper Aldi Haxia's goal from a tight angle, his second came when Michael Coulson whipped in a cross that wasn't cleared, Hearn unleashed a superb right-footed shot on the turn, across goal and in off the far post for his 10th league goal of the season.

Grimsby's biggest win of the season came on 29 November 2011 in a 7–0 thrashing over Stockport County, Hearn scored his third hat-trick, on 22 minutes Grimsby were 2–0 up when Jamie Green from inside the box squared to the unmarked Hearn who scored a tap-in to the bottom left corner of the goal inside the area, on 45 minutes Hearn put Grimsby 5–0 up when Anthony Elding found Hearn who struck from 10 yards out, on 58 minutes Jamie Green took a corner which was met by Charlie I'Anson providing the knockdown to Hearn who then rifled into the roof of the net from a few yards for the 6th goal.

On 17 December 2011, Hearn scored a double in the 4–3 home win against Ebbsfleet United, in the opening 2 minutes latching on to a poor back pass, racing away and firing past their keeper, he scored the third goal for Grimsby on 13 minutes, Luke McCarthy's mistake, slipping a back pass through to Hearn who was composed and clinical as he shot beyond the keeper from inside the penalty area for 3–0.
On 26 December 2011,  Hearn scored the crucial winning goal to put Grimsby 2–1 up in the derby against Lincoln City, Manny Panther combined with Craig Disley who then flicked it over the defence for Hearn to accelerate into the box, keeping his composure to fire under Farman and into the left corner of the goal.

The following calendar year he helped Grimsby do the double in a 3–1 home victory over rivals Lincoln City scoring the opening goal on 25 minutes, Conor Townsend beating the opposing defenders to curl in a cross from the left through to Hearn who lashed it home from eight yards.
The following Saturday on 8 January 2012 he scored twice against former club Alfreton, Townsend with a perfect long ball to Hearn who cut off the defence to shoot Grimsby ahead on 11 minutes with a low effort from the edge of the penalty area, a quality through ball fr
Hearn signed a three-year contract with Grimsby Town on 17 June 2011, for a £30,000 transfer fee. His first goal for Grimsby came 11 games into the 2011–12 season against Wrexham, a through ball from Craig Disley firing a low shot passed Chris Maxwell. The following week he scored 4 goals against his former club Alfreton on 1 October 2011, scoring the equaliser on 57 minutes finishing from close range. His second came 3 minutes later when Duffy fed Hearn a  ball over the top to shoot past the keeper, the hat-trick goal came in the 70th minute, firing a shot the keeper could only assist into the net. Hearn's fourth goal of the afternoon came in the 86th minute when he took the ball past the defence and the keeper to slide home and put Grimsby 4–1 up, the game finished a 5–2 victory to the Mariners. Two games later on 11 October 2011, Hearn scored his 2nd hat-trick of the campaign in a 5–2 win over Barrow, he scored the first after racing clear of the defence to shoot underneath the keeper, after Barrow equalised, Hearn scored his second in first-half stoppage time, a one-two with Craig Disley put him in on goal holding off his marker, he side-footed home this time with his left-foot, his third came on 75 minutes, muscling his way through to turn in the box and beat Hurst with a low shot from six yards.
Hearn helped Grimsby to a 5–0 rout over Ashington in the fourth qualifying round of the FA Cup on 29 October 2011, Coulson and Duffy combined on 38 minutes to put Hearn through, who skipped round Dryden the keeper, before his first shot effort was well saved by Dryden, but he made no mistake at the second attempt; two minutes later another ball from Robert Duffy to Hearn who then muscled through the defence, scoring past Dryden from 10 yards.
On 26 November 2011, he scored two more goals in a 2–1 victory at Kettering, scoring the first on 14 minutes when Hearn read a lofted ball forward and muscled past defender Jamie Navarro before slotting past goalkeeper Aldi Haxia's goal from a tight angle, his second came when Michael Coulson whipped in a cross that wasn't cleared, Hearn unleashed a superb right-footed shot on the turn, across goal and in off the far post for his 10th league goal of the season.
Grimsby's biggest win of the season came on 29 November 2011 in a 7–0 thrashing over Stockport County, Hearn scored his third hat-trick, on 22 minutes Grimsby were 2–0 up when Jamie Green from inside the box squared to the unmarked Hearn who scored a tap-in to the bottom left corner of the goal inside the area, on 45 minutes Hearn put Grimsby 5–0 up when Anthony Elding found Hearn who struck from 10 yards out, on 58 minutes Jamie Green took a corner which was met by Charlie I'Anson providing the knockdown to Hearn who then rifled into the roof of the net from a few yards for the 6th goal.
On 17 December 2011, Hearn scored a double in the 4–3 home win against Ebbsfleet United, in the opening 2 minutes latching on to a poor back pass, racing away and firing past their keeper, he scored the third goal for Grimsby on 13 minutes, Luke McCarthy's mistake, slipping a back pass through to Hearn who was composed and clinical as he shot beyond the keeper from inside the penalty area for 3–0. On 26 December 2011,  Hearn scored the crucial winning goal to put Grimsby 2–1 up in the derby against Lincoln City, Manny Panther combined with Craig Disley who then flicked it over the defence for Hearn to accelerate into the box, keeping his composure to fire under Farman and into the left corner of the goal.
The following calendar year he helped Grimsby do the double in a 3–1 home victory over rivals Lincoln City scoring the opening goal on 25 minutes, Conor Townsend beating the opposing defenders to curl in a cross from the left through to Hearn who lashed it home from eight yards. The following Saturday on 8 January 2012 he scored twice against former club Alfreton, Townsend with a perfect long ball to Hearn who cut off the defence to shoot Grimsby ahead on 11 minutes with a low effort from the edge of the penalty area, a quality through ball fr
Liam Hearn