The cowardly political assistant hid behind an online alias to spread vile lies about a string of opponents.
The disgraced Nat - who has now been sacked as Constitution Minister Mike Russell's constituency office manager - even targeted Gordon Brown with a sickening mocked-up picture of the Prime Minister being EXECUTED.
And the scandal now threatens to overshadow tomorrow's announcement by First Minister Alex Salmond of his White Paper calling for a referendum - which Russell has overseen.
An SNP insider said: "It's totally unbelievable that a guy working for him would risk derailing that.
"You've got to wonder at Mike hiring this guy. Alex was looking to him to make sure the white paper launched smoothly. That's gone now."
MacLachlan, 46, used the online name Montague Burton to spread dozens of lies, including:
VILE and untrue allegations about Scottish Labour's General Secretary Colin Smyth bullying women
FALSE claims branding top QC and Tory Paul McBride "the biggest liar in the Scottish legal system", and
VICIOUS lies about a married male Labour MSP trawling Edinburgh for gay sex.
MacLachlan, who worked in Russell's Dumfries office, also targeted a string of female Labour politicians with offensive and personal abuse.
And he taunted former First Minister Jack McConnell, branding his Motherwell and Wishaw constituency a "s***hole".
Hiding behind his internet identity, shamed MacLachlan also used his blog to launch a personal attack on the integrity of Professor John Curtice, a politics expert at Strathclyde University.
Smug MacLachlan thought his bitter hate campaign would never be traced back to him because he always used his pseudonym.
But a News of the World probe this week unmasked him. And within minutes of contacting the Government, MacLachlan - who picked up £30,000 a year of taxpayers' cash - had been forced out of his post.
A source said: "Mark is a failed journalist. He wanted to be a real campaigning reporter but he ended up being Mike Russell's bag carrier.
"Now he's thrown that away. He'll never work for the SNP again. Alex Salmond will never forget. But Mark's only got himself to blame."
In his vile blog, which was pulled from the internet on Friday night, MacLachlan falsely accused Dumfries and Galloway councillor Smyth of getting a "thrill" from "bullying and dominating the ladies".
A Labour spokesman confirmed Smyth now planned to sue over the smears. And Paul McBride QC, who is also considering legal action, added: "This individual has spent years abusing and defaming me.
"I am delighted he has been exposed and shocked to learn he was a minister's aide."
MacLachlan also targeted journalists and broadcasters - causing more embarrassment for South of Scotland MSP Russell, whose ministerial brief includes culture and the media.
As well as writing his blog - called the Universality of Cheese - the shamed aide posted abuse and lies on a range of other websites and newpaper comment pages.
Opposition leaders last night united in condemnation of MacLachlan's twisted online campaign - as Lib Dem leader Tavish Scott called on Salmond to SACK Russell if the blogger posted smears while at work.
Labour leader Iain Gray said: "Alex Salmond was recently urged to 'fight dirty' by former SNP leader Gordon Wilson and now we see what they mean.
"The First Minister has to make clear if Mike Russell knew about his aide's blog or not. By his silence Alex Salmond condones it.
"This long-running hate campaign by the SNP has no place in Scottish politics but he has done nothing about it."
Lib Dem leader Scott said: "We need an apology from Alex Salmond and for him to dissociate himself from this blogger immediately.
"This poisonous SNP bile, attacking anyone who dares criticise Alex Salmond's nationalists, was paid for by the Scottish taxpayer.
"I want a full investigation by the Permanent Secretary into what other SNP special advisors, researchers and hired hands are involved in this kind of odious politics."
Scots Tory chief Annabel Goldie added: "This politics of vile hatred is abhorrent. Behind the smiles, the nasty underbelly of the SNP has been exposed.
"At a time when trust in politics had never been lower, this is more than just an embarrassment for Alex Salmond as he launches his doomed independence bill."
Salmond was said to be FURIOUS over the scandal and had demanded MacLachlan's immediate dismissal.
The SNP yesterday issued a grovelling apology from the sacked office manager.
He said: "I deeply regret my actions and unreservedly apologise to all the people concerned and to Mike Russell.
"In getting carried away with my comments, which were very wrong, I have let myself and my colleagues down, and of course I have resigned from my potision in Dumfries."
A spokesperson for Russell added: "Mike was shocked to be told about this.
"He knew absolutely nothing about it and has accepted Mark MacLachlan's resignation."