Do Pokemon get there ability effects on the first turn the mega evolve?

Question

if my Prankster Sableye mega-evolves and uses Substitute, will the Prankster priority activate on the first turn, or will it act as usual?

Answer 1

Changes in a Pokémon's Ability will also take place immediately. A Sharpedo with Speed Boost will not gain a boost in speed at the end of the turn it Mega Evolves, but if it uses a biting move such as Crunch that turn, it will be boosted by Strong Jaw, Mega Sharpedo's Ability.

This is a section from Bulbapedia's article on Mega Evolution.
So yes, the Mega Evolved Pokémon will get their ability on the first turn :)

Comments

Your Mega Sableye will not use Substitute first unless it outspeeds

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Just saying, your comment about Sableye is wrong. Anything Speed-related stays the same as the original form for the first turn. Prankster counts as Speed-related, so Mega Sableye retains both Prankster AND Magic Bounce on Turn 1. Abilities such as Speed Bosost, however, are replaced once Mega Evolving since they do not actually increase Speed, per se, they merely boost it whilst the Pokémon is on the field.

Source: http://pokemondb.net/pokebase/226291/mega-sableye-retaining-prankster and experience.

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Oh right so just speed related ones stay on just the first turn?

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Basically it's just Prankster and Prankster only.

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okay thanks

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if i megavolve my banette will she gain confuse ray priority?

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Not on the first turn. Prankster doesn't take effect until turn two.