Lord Watson, 56, a former tourism, culture and sports minister in the Scottish Parliament, had developed drink problems linked to difficulties at home, Edinburgh Sheriff Court was told. He committed his arson attack after the Scottish Politician of the Year awards ceremony last November.
The blaze broke out in the main reception of the Prestonfield House Hotel in Edinburgh at 2.15am on 12 November. Watson initially denied starting the fire, which caused £4,500 worth of damage, but CCTV footage showed him crouching at the base of a curtain minutes before it was engulfed in flames.
Lawyers for the peer said his wife, Clare, had conceived after IVF treatment but he plunged into depression after the loss of the unborn baby months later and started drinking heavily.
Lady Watson was in tears as her husband was led away to begin his term. Outside court, a spokesman said Watson wanted to pay tribute to his wife and those who had stood by him.
Paul Burns, a lawyer for the peer, said: "Although he is a robust and strong person, he is a human being and I suspect that took him to a lonely and dark place."
Mr Burns told the court the events of that night led to the "spectacular destruction of [Lord Watson's] public life". He added: "The psychological report clearly indicates alcohol had become problematic for him and it seems to suggest that, like a lot of people, he may, indeed he has, crossed the line that separates alcohol from recreation into problem."
The lawyer, who said he sought no special treatment for his client, said the pressure of living life in the public gaze was one which others may struggle to understand. He spoke of Watson's sadness at no longer being part of the Scottish Parliament, having stood down from his post on the day he pleaded guilty to one charge of wilful fire-raising.
Mr Burns said: "It is a particularly painful thing for him to find himself in this present situation, debarred at his own hand from that institution that represented so much of his life's work."
The Sheriff, Kathrine Mackie, said although Watson was drunk, that "neither excuses nor fully explains" his behaviour. She said jail was the only option because there was a significant risk that the former Glasgow Cathcart MSP would reoffend. She said: "Someone in public office ought to know how to conduct himself on all occasions."
Branding his behaviour "deliberate and dangerous", Ms Mackie continued: "Fire-raising is a serious crime. The potential for serious injury to guests and staff within the hotel, and for very significant damage to the property, was considerable."
A Labour spokesman confirmed Watson had been expelled from the party. He said: "He has [also] lost his seat in the Scottish Parliament, his career and his reputation and now, having already paid a heavy price, he has been jailed. His sentence illustrates that if you commit a serious crime in Scotland, no matter who you are, you must face the consequences. That is right."
A by-election will be held in Watson's former constituency next week.
* Jamie Blandford, Marquess of Blandford, the son of the Duke of Marlborough, has been to prison three times and fought a lengthy battle against drug addiction. The Duke and trustees of the ancestral Blenheim estate in Oxfordshire took legal action to ensure Jamie would not inherit control of the estate. Blandford will still be able to live at Blenheim and inherit the title.
* Jeffrey Archer was ennobled for services to the Tory party. He lost his job as deputy chairman of the party in 1986 after a tabloid newspaper published allegations about hsi relationship with a call girl. He successfully sued for libel but was jailed in 2001 after a jury found that he committed perjury. He served two years of a four-year sentence but the conviction did not cost him his peerage.
* Lord Brockett was sent to Ford Open Prison in the 1990s for a £5m classic cars insurance fraud. The Old Etonian celebrated his release from jail in 1998 with a society barbecue. He then became one of the stars of the 2004 version of the reality TV show I'm A Celebrity, Get Me Out Of Here! alongside Jordan, Peter Andre, the ex-Sex Pistol John Lydon and the winner, Joe Pasquale.Reuse content