The Ultimate Thor from Marvel Earth #1610 was believed to be wielding a version of Mjolnir but it is revealed the hammer is not actually the magical device known in the Marvel Universe #616.
In the Ultimate Marvel imprint title the Ultimates' first two series, the alternate universe version of Thor wields a weapon that while called Mjolnir is a hybrid of hammer and axe.
The Ultimate Thor entry on Wikpedia points out:
However, the ax/hammer hybrid primarily wielded by the character is
considered to be the "Ultimate" version of Mjolnir, as revealed in
Ultimate Power #9. However in the Ultimate Thor miniseries it is
revealed that the original axe hammer is no longer necessary, as it's
not the real Mjolnir but instead, Dr. Braddock gives Thor tech-armor,
and Thor suggests to turn the power supply into a hammer. It is stated that this version of the hammer is not restricted by the worthiness test.
Even when Thor wielded the true hammer Mjolnir, the Earth #1610 version did not have an enchantment making it unable to be used by the unworthy. It was stolen and used several times by various villains, Magneto, Gregory Stark and by Captain America. When the Children of Tomorrow slaughtered the Asgardians, triggering the second Ragnorok, Mjolnir was hidden with Thor's son Modi in the Room with No Doors, in an attempt to keep it safe until Asgard may be restored to its former glory. Thor returned to Earth and he was given a Stark-enhanced version of the EDI power harness and the axe-hammer he wielded previously.
Stark-enhanced EDI Harness and Techno-Mjolnir