The Queen's title was won by Matteo Berrettini for the second year in a row.
The second seed from Italy put in a strong serving performance to beat the Serbian player.
Berrettini has won 20 of his past 21 grass-court matches, with his only loss coming against Novak Djokovic in the Wimbledon final.
I want to cry because there's too many emotions.
The world number 10 has won back-to-back titles after defeating Andy Murray last week.
After having surgery on his right hand, Berrettini only returned to the tour last week and admitted he had not been sure what to expect.
He roared to the crowd after he converted his first match point with an ace.
His team covered him in water as he celebrated with his father, who was in London for the match.
Berrettini said he forgot how heavy the trophy was.
I didn't think I'd ever win two titles in a row after surgery.
Every time I walk in the hallways, I see all the names of the champion from the past, and now that I know it's me, twice, on the same wall, it gives me goose bumps.
He will be one of the favorites at Wimbledon.
He will leave Queen's Club with some positives after failing to win a main draw match on grass before this week.
He hated playing on grass before this tournament.
I feel like I want to play more and more, so we'll see how it goes."
Thank you very much for helping me, I felt like I was at home.
A heavy favorite for the match given his grass-court credentials, Berrettini started well and took the fifth with a backhand volley.
Krajinovic hit a volley of his own to get the break.
The two were evenly matched but with Berrettini having such a strong serve, the pressure was always on the Serb.
A netted backhand from Krajinovic gave Berrettini the break for 5-5 and he never looked back, with a huge forehand winner into the corner of the court securing the set.
The Italian kept the pressure on in the second, breaking to love for a 3-2 lead, but his nerves got the better of him as he served for the match.
He raised his arms in triumph as his victory was confirmed.
There was disappointment for Lloyd Glasspool as he and his partner lost the men's doubles final to the Serbians.
The wheelchair doubles title was won by the Hewett and Houdet duo.
In the Halle final, Hurkacz beat the world's number one ranked player.