Arsenal had to beat Liverpool by two clear goals to take the title. They were 16-1 outsiders at the start of the season and Liverpool were the reigning champions, unbeaten at home in the league since January.
Wengrow, however, was much closer to the Arsenal team than the average supporter. The night before the decisive game, he was at David O'Leary's house.
"I was actually in the team hotel for most of the day when they played at Anfield," Wengrow told Football Presse.
"The night before the game I was over at David O'Leary's house."
That tells you something about the relationship Wengrow had with Arsenal's players. He wasn't simply someone who watched them from the stands. He was part of their circle. And the following morning, he was around the team again.
"The team travelled up at 10 o'clock the next morning. That can never happen today."
It was an extraordinary decision by George Graham. Arsenal normally would have travelled the day before an away match, but Graham chose to take his players north on the morning of the biggest game of their season.
Wengrow remembers how unusual the whole thing felt.
"The players had to get out of bed early in the morning to be ready, and I agreed to get the team coach leaving at 10 o'clock.
"All of it strange."
But perhaps the strangest thing was that nobody around Arsenal seemed to believe the impossible was actually going to happen.
"I backed Arsenal at the start of the season at 16-1 to win the league.
"And when they played at Liverpool in that one game, they were still 16-1, the exact opposite.
"They were 16-1 outsiders."
Arsenal had reached the final game knowing exactly what they needed to do. A draw would give Liverpool the title. A one-goal Arsenal win would not be enough. Yet Wengrow remembers remarkably little sense of panic around the Arsenal camp.
"Nobody thought Arsenal could do it. I was at O'Leary's. There was going to be a sting in it."
Liverpool, too, seemed caught between the need to protect their advantage and the knowledge that Arsenal had nothing to lose.
"It was a very relaxed atmosphere if you think about it. Nobody thought Arsenal could do it."
The first half finished goalless.
That was exactly what Graham had wanted. Liverpool had the title. Arsenal had 45 minutes to somehow change everything. Then came Alan Smith's goal early in the second half, putting Arsenal 1-0 ahead.
Suddenly the impossible was one goal away. But Wengrow remembers the strange psychology of the game.
"Liverpool didn't play their normal game. It was like they didn't know whether to stick or twist.
"They didn't need to.
"A 0-0 draw, they'll win the league. A 1-0 defeat, they'll win the league."
The clock continued to run. Arsenal needed one more goal. And then, deep into stoppage time, Michael Thomas broke through.
Wengrow has watched enough football to understand just how improbable what followed was.
"Who wins 2-0 at Anfield, especially in a game of that magnitude? It just doesn't happen."
"But it did."
Thomas slipped the ball beyond Bruce Grobbelaar and Arsenal had won the championship on goals scored, ending an 18-year wait for the title. The goal came with virtually the last kick of the season.
For Wengrow, though, the importance of the night wasn't simply that Arsenal had won the league. It was the way they had done it.
"It's one of the reasons why we follow football. We stick through 99 rubbish games for experiences like that.
"It's an incredible game, isn't it?"
It was also another extraordinary chapter in a relationship that had already taken Wengrow inside the Arsenal dressing room for some of the club's greatest moments.
He had been around the players before the game. He knew many of them personally. And now he was witnessing one of the most famous finishes in English football history alongside people he regarded as friends.
"I've been lucky because I've been at all the games. I've been able to enjoy every single Arsenal success over these last 60, 70 years.
"And I've been present to witness them."
Anfield was different, though. Arsenal had not simply won a title. They had gone into the home of the champions needing the most unlikely result imaginable and produced it.
"That's why we follow football.
"That's why we stick through 99 rubbish games for experiences like that."
For Wengrow, 1989 remains the perfect example of why a lifetime spent following Arsenal has been worth it.
Nobody expected them to win. They needed two goals. They scored two goals.
And Michael Thomas wrote the final line.