Maharoof posted figures of 3-24 and Lokuarachchi 3-47, but after restricting Pakistan to 201-8 in their 50 overs, the home side were denied a chance to chase the target by heavy showers here on Friday.
Electing to field on a slow pitch at the Premdasa Stadium, the Sri Lankan bowlers bowled tightly and restricted the visitors to a low total.
Maharoof struck twice in successive overs to see the backs of opener Kamran Akmal (five) and Shoaib Malik (seven) who were both bowled with deliveries that cut back sharply.
The experienced duo of captain Inzamam-ul Haq and Mohammad Yousuf fought back with a 53-run partnership off 68 balls for the fourth wicket but then fell in quick succession.
Maharoof removed Inzamam (24), who played a loose shot to a wide ball outside off and wicketkeeper Kumar Sangakkara took a fine catch diving to his right.
After Maharoof's strikes, leg-spinner Lokuarachchi took the important wickets of Yousuf, hard-hitting batsman Shahid Afridi (two) and all-rounder Abdul Razzaq (32).
Younis Khan (41) battled hard towards the end but the runs never flowed.
The remaining two matches of the series will also be held in Colombo. A two-Test series with matches in Colombo and Kandy will follow the One-dayers.