The former Stockport County and Mansfield Town midfielder scored the opening goal of the game as the Shrimps, reduced to ten men early in the second half, bounced back from Saturday's miserable 4-0 drubbing away to Crawley Town.
And to insult to injury Danny Carlton scored a killer second goal with ten minutes remaining to silence the home team.
A pinpoint pass from David Perkins found Carlton in the 23rd minute and, after Altrincham keeper Richard Acton had produced a superb point-blank save to block the big striker's left foot strike, it was McLachlan who was on hand to head home from ten yards out.
But the visitors were made to graft for the points, battling for over half an hour with one man short after Gary Thompson's red mist threatened to hand the Robins a lifeline.
In the 59th minute the winger clashed with centre-back Karl Munroe. The Alty player received a yellow card for his part in the fracas, which saw both sets of players surround referee Ian Smedley.
However, Thompson was shown a straight red after looking like he retaliated after being sandwiched by two defenders in the home side's penalty area.
Then, as the home team pushed forward for an equaliser, Carlton squeezed between two defenders to tap in Adam Yates' cross from the right wing.
Earlier, goalscorer McLachlan shaved the woodwork with a cross-shot which had Acton back-pedalling nervously.
And Morecambe were good value for their goal, dominating the early exchanges as Graham Heathcote's team failed to impress in front of the home fans.
The visitors had a penalty appeal turned away by official Smedley in the 17th minute after centre-back Steve Rose looked to have caught winger Thompson.
Thompson had raced past Warren Peyton at left back, thought about shooting once inside the box, but then went to ground and Morecambe protested their man had been felled.
But the pressure told and six minutes later Perkins and Carlton combined, leaving McLachlan to net the opener.
At the other end Val Owen had Altrincham's best chance, but his dipping right-foot shot flew agonisingly over the crossbar.
Moments later the midfielder found time and space in front of goal, but from Peter Band's lay-off fired his left-foot shot wide of the right-hand post.
McLachlan set up Perkins five minutes before the break, only for the midfielder to be denied by a brilliant block by the on-song Acton.
Sammy McIlroy's team nearly doubled their advantage just 53 seconds after the restart.
Goalscorer Carlton hit the left-hand post with a sweet left-foot strike, but the rebound fell to McLachlan, who won a free-kick on the edge of the box after he was felled needlessly.
It was Thompson who took the free-kick, but Acton made a smart stop to deny him, getting down low to block.
Perkins crossed from the left in the 51st minute and Craig Stanley fired over from distance with an ambitious half-volley, before Thompson was sent off and Altrincham sensed a comeback.
But substitute Pat McFadden saw a shot charged down in the 65th minute and Altrincham were then left gobsmacked when Carlton sneaked in to secure the victory.
McLachlan had a good chance late in the action as Morecambe stayed alive, but Alty were left to contemplate what might have been after a poor home performance.