*Counterspell* states:

> You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. 

This wording suggests that a successful *counterspell* interrupts the spell, causing it to fail. 

The Mage Slayer feat (PHB, p. 168) states:

> When a creature within 5 feet of you casts a spell, you can use your reaction to make a melee weapon attack against that creature.

This text seems to say that you only get the attack if the creature finishes casting the spell. 

**If a spell is counterspelled, does it still count as a spell cast for the purpose of Mage Slayer's reaction attack?**

The answers to [this question][1] seem to suggest yes, but unlike many other interrupts, *counterspell* does actually prevent the spell from being cast, so it's a bit unclear. 

  [1]: https://rpg.stackexchange.com/questions/86783/does-a-countered-spell-cost-a-spell-slot