Harmison's six for 19 in just 13 overs saw Pakistan bundled out for a mere 119 on the first day here at Old Trafford, England seizing the initiative in this four-match series after the drawn first Test at Lord's.
At stumps England were 168 for two, a lead of 49, with Alastair Cook 65 not out and Kevin Pietersen 38 not out after Harmison had exploited a hard, bouncy pitch at Old Trafford following Pakistan captain Inzamam-ul-Haq's decision to bat first on winning the toss.
And the England spearhead was particularly proud to have given captain Andrew Strauss the kind of performance he felt his skipper was entitled to after some mediocre efforts during the 5-0 one-day series thrashing by Sri Lanka, where he was coming back from a shin injury.
"If we get another day like today (Thursday), there should only be one winner," Durham quick Harmison said after a Pakistan collapse that saw the tourists bowled out inside 39 overs.
Strauss, leading the team in the absence of injured all-rounder Andrew Flintoff and crocked Ashes-winning captain Michael Vaughan, was widely criticised for delaying a declaration until after an hour's play on the final day at Lord's.
"'Straussy' was looking for his senior bowlers to perform for him," Harmison explained. "I didn't let him down during the one-dayers because I gave him everything I had, I just wasn't good enough.
"I feel as though I gave him everything I had at Lord's but I'm pleased for him today because everything I had was good enough.
"And that's what he needed, he needed somebody to stand up for him after he got harshly criticised after Lord's."
Harmison's figures were his best in Test cricket since he took a stunning seven for 12 against the West Indies two years ago at Jamaica's Sabina Park.
"There was more rhythm at Sabina Park but today my action was solid compared to last week. You could count on one hand how many balls went down legside. I think I had an idea where it was going this time," the 27-year-old added.
"I feel as though I came here with more miles in my legs and a bit of a spring in my step.
"At Lord's I did feel a bit heavy-legged, a bit tired, which was understandable because I hadn't spent a day in the field since I left India.
"I worked very hard with Kevin Shine (England's bowling coach) in the two days prior to this Test match and everything has gone brilliant because I've got six for 19.
"I've been waiting eight months for that pitch."
Harmison was well-supported by left-arm spinner Monty Panesar who achieved both sharp turn and control on his way to a fine haul of three for 21 in 7.4 overs.
Pakistan vice-captain Younis Khan, who top-scored with 44 in an innings where Mohammad Yousuf's 38 was the only other notable score, said the pitch had played its part in Harmison's success.
"Harmison bowled well but in the first session it was a little bit dodgy with a lot of uneven bounce," said Younis, whose side were without their three leading fast bowlers - Shoaib Aktar, Mohammad Asif and Rana Naved-ul Hasan because of injury.
"He utilised the cracks very well."
But Younis insisted the match was far from over.
"We have four more days. England have to bat last and if we leave them 200 to win it will be very difficult for them."