MTM 2020 Update
In response to the coronavirus pandemic and the resulting employment and economic situation, IRS postponed the filing of many tax returns and elections.
This postponement is detailed in IR Notice 2020-23, a copy of which can be viewed here.
In that Notice, IRS states:
... elections that are made or required to be made on a timely filed Specified Form (or attachment to a Specified Form) shall be timely made if filed on such Specified Form or attachment, as appropriate, on or before July 15, 2020.
Since a Mark-to-Market election is made by attaching the election to a Form 1040, 1041, 1065, 1120, or 1120-S, it would seem to state that the MTM election is valid if attached to a tax return or extension due by July 15, 2020.
However, in the case law dealing with the MTM election and late filing for it, the courts have routinely stated that taxpayers cannot have the benefit of hindsight in order to make the election, which typically jeopardizes the position of the government.
But these are strange times ... so the election should be made ... there is nothing to lose.
What if you trade inside an entity and that entity's tax return was due on March 15 - Form 1065 or Form 1120-S - and you did not attach the election? I believe you are simply out of luck in that situation since that filing deadline was not extended.