Darekar met party president Raj Thackeray and handed over his resignation.
Thackeray's decision on Darekar's resignation letter was not yet known.
Darekar was an MNS legislator who lost from Magathane constituency in northwest Mumbai to Shiv Sena's Prakash Surve.
A popular sitting legislator, Darekar was pushed to the third position behind Bharatiya Janata Party's Hemendra Mehta.
He was apparently peeved at a comment by party colleague Bala Nandgaonkar, who reportedly said MNS legislators lost because they did not perform well in the past five years.
Darekar was among the earliest and closest confidantes of Raj Thackeray, and the duo had been working earlier with the Shiv Sena since 1987.
When Raj Thackeray quit the Shiv Sena in 2005, Darekar followed him and was made a key functionary to build up the MNS which sent 13 legislators to the 12th Maharashtra assembly in the 2009 elections.
In the recent assembly elections, the MNS was reduced to just one legislator from Junnar, Nashik, raising question marks on the very survival of the party which vacillated between praising and opposing Prime Minister Narendra Modi and waged regular wars against north Indian migrants.