His nearest rival is Nai Khan Ari, an Independent, who received 494, followed by Aminah Abdullah, a former PKR Penang Wanita chief who stood as an Independent, with 392. Kamarul Ramzu received 56 votes.
Mansor's majority is 5,558 votes. All the three independents lost their deposit.
This is the sixth by-election since Election 2008. BN has not won in any of the four previous by-elections in Peninsular Malaysia, only winning in Batang Ai in Sarawak.
Another by-election will be held in the rural Kelantan state seat of Manek Urai on July 14 after its five-term assemblyman Ismail Yaacob from PAS died on May 22.
Report by star online
Penanti: Mansor gets 6,052 votes out of 7,100 as of 7.17pm (Update 6)
BUKIT MERTAJAM: At 7.17pm PKR's Mansor Othman has won 6,052 of the 7,100 votes cast in the Penanti by-election.
He is trailed by Independents Nai Khan Ari (494), Aminah Abdullah (392) and Kamarul Ramizu Idris (56).
Mansor looks all set to become the next Deputy Chief Minister 1.
Polling ended at Penanti on Sunday with a low turnout of 46.1% or 7,100 voters despite fair weather.
Penanti has 15,384 registered voters.
Results are expected to be announced by 8.30pm.
Election Chairman Tan Sri Abdul Aziz Mohd Yusof said he was disappointed by the low turnout.
“We do not why but we will study the reasons for it,” he said, adding that he had expected a 70% turnout.
The usual buzz and excitement that surrounds by-elections was missing this time as Barisan Nasional had declined to contest.
The Election Commission had strictly enforced the no "pondok panas" rule within 50m of the polling stations, which meant there was no last-minute wooing of voters.
At about 3pm Sunday, 42.2% or 6,492 voters had cast their votes at the nine polling stations.
Although the weather was fine there was only a trickle of voters and traffic flow was smooth.
However, both FRU and police personnel remained on standby for any eventuality. No such eventuality has arisen.
Voters in Penanti comprise 72.68% Malays, 24.44% Chinese, 2.39% Indians and 0.71% of other races.
It was a four-cornered fight with PKR's Mansor Othman taking on three Independents - former Penang PKR Wanita chief, manufacturing manager Nai Khan Ari, 41, and businessman Kamarul Ramizu Idris, 42.
Among the earliest to cast his vote was Nai-Khan Ari at the Kg Tun Sardon community hall at 8.40am.
Earlier, Dr Mansor had visited the Berapit polling station with DAP's Lim Kit Siang and Penang Chief Minister Lim Guan Eng.
Aminah has recently added in a twist to the otherwise peaceful campaign period by claiming that her former party has offered her RM80,000 and the Penang Municipal Council president's post if she withdrew from the contest.
She also alleged that PKR leaders had offered her the Deputy Chief Minister's post if she won.
She has since lodged a report with the Election Commission.
The Penanti seat fell vacant after Mohammad Fairus Khairuddin quit as assemblyman on April 16 after stepping down as Penang Deputy Chief Minister I on April 8.
In the last general election, PKR's Fairus polled 7,346 votes against the 5,127 votes obtained by his Barisan opponent Datuk Abdul Jalil Majid. There were 184 spoilt votes .