The 26-year-old emerged triumphant after a 6-7 6-4 6-4 6-3 victory to reach his third consecutive Grand Slam final.
Murray would become the first Briton to win the men's singles Championships at SW19 since Fred Perry in 1938 if he defeats Novak Djokovic on Sunday.
Djokovic defeated Juan Martín del Potro in an epic five-setter which finished just shy of five hours.
Just like against Fernando Verdasco on Wednesday, Murray was downed in the first set as Janowicz's formidable serve belied his Grand Slam semi-final inexperience. The world number 22, who has served the fastest this Championships, fired deliveries at 140mph and his opponent was comfortably vanquished in the tie-break.
Murray replied swiftly when Janowicz was broken in the first game of the second set, double-faulting at 30-40 after his decision to go for a powerful second serve backfired.
Things became fraught when Murray raged at Wimbledon officials after they decided to pull the roof over Centre Court when he was one set from victory.
Having come back from 1-4 down, Murray stormed his way through the remainder of the third to win it 6-4, however Janowicz had started complaining about the light, although those complaints stopped as soon as he gained a lead.
When Murray regained the initiative, the Pole remonstrated with the umpire. Following the end of the third set, tournament referee Andrew Jarrett told the players the roof would be closed and the lights switched on, much to Murray's annoyance.
Murray was overheard on courtside microphones saying: "How long has he been complaining about the darkness? It's not even dark."
The Centre Court crowd also objected, chanting: "Stay on, stay on."
Following a 20-minute interval, Murray banished his concerns in just the third game to break the 6ft 8in Pole, who was earlier subjected to boos from the patriotic spectators.
Janowicz was bothered and Murray was buoyed in the indoor surroundings. Murray held comfortably and ultimately broke Janowicz again to seal his berth in Sunday's finale.
"I'm delighted," Murray told the BBC. "It was a very tough match and completely different to any other match I've had here this year. He's talented and unpredictable, he has huge serves, which give you very little rhythm come back at him."
On the decision to close the roof, he added: "It's a tough situation, there was about 45 minutes of daylight left. It's an outdoor event and we should play as much outdoors as we can. And I'd won five games in a row. I took a shower, spoke to the guys and got to back work.
"He (Janowicz) was on his phone at the break, he seemed very relaxed. That's the sort of player he is, that's why he is very loose on the court."