In the film Thor, Odin casts a spell on Mjolnir and strips Thor of his power. He then banishes Thor to Midgard (Earth). On Earth, Thor is seemingly just a strong and durable human. Later in the film, he becomes worthy, and regains control of Mjolnir. This then grants him his full power set and control over Mjolnir.
My question is, prior to this, could Thor control lightning all on his own? Or is this purely an ability of Mjolnir?
As the Norse god of thunder, Thor can summon the elements of the storm (lightning; rain; wind; snow) and uses Mjolnir as a tool to focus this ability, although the hammer cannot command artificial weather, only natural. He can cause these weather effects over the world and destroy entire buildings; by whirling his hammer he can lift entire buildings with the wind. As the son of the Earth goddess Gaea, Thor has shown some control over the Earth.
REF: Lee, Stan; Bernstein, Robert (w), Sinnott, Joe (p), Sinnott, Joe (i). "Thor and Loki Attack the Human Race!" Journey Into Mystery 94 (July 1963)
That said, in the Marvel Universe, Odin possesses the ability to withdraw greater powers from Thor (and likely any Asgardian) he wishes since all of their powers and abilities come down to them through Odin the same way Odin acquired his powers from his brothers Velli, Ve and and their father Bor.
When Odin banishes Thor to Midgard, on more than one occasion he has withdrawn some or even all of Thor's abilities depending on his level of pique. When Thor is first banished to Midgard for arrogance in Journey into Mystery, he is in the form of the handicapped Doctor Donald Blake and unaware of his divine heritage or gifts.
Thor's father Odin decides his son needed to be taught humility and consequently places Thor (without memories of godhood) into the body and memories of an existing, partially disabled human medical student, Donald Blake.
ODIN: "You! Lack! Humility! And humanity! You are supreme in your power and your pride! The lessons you need so profoundly can never be learned by a god of thunder -- thus I cast you out -- thus shall you shed your godly trappings -- no longer art thou God of Thunder! Thy memory too shall I strip bare! Then go -- for life anew awaits -- on Midgard..."
After becoming a doctor and on vacation in Norway, Blake witnesses the arrival of an alien scouting party. Blake flees from the aliens into a cave. After discovering Thor's hammer Mjolnir (disguised as a walking stick) and striking it against a rock, he transforms into the thunder god.
- Later, in Thor #158, Blake is revealed to have always been Thor, Odin's enchantment having caused him to forget his history as The Thunder God and believe himself mortal.
Thor: Ragnarok spoilers ahead:
Although even Thor never realized it, he can control lightning on his own.
During Thor: Ragnarok, after Mjolnir is destroyed, Thor occasionally exhibits some glimmer, or I should say sparkle of ability. But at the climax of the movie, Thor has a vision of Odin, which tells him "That hammer was to help you control your power; focus it. It was never your source of strength." This realization allows Thor to fully use his power to then defeat Hela and save the day.
''are you the god of hammers? the hammer was to help you focus your power it was never your source of strength'' - Odin
