Yes, as long as the mount is within 5 feet
An ally is not clearly defined in 5e, so we can use the English definition:
a person, group, or nation that is associated with another or others for some common cause or purpose
Ally in the case of D&D 5e is almost certainly meant to include animals since wolves get the Pack Tactics trait too so we just have to assume the mount has "a common cause or purpose"
Your steed serves you as a mount, both in combat and out, and you have an instinctive bond with it that allows you to fight as a seamless unit.
(emphasis mine) fighting as a seamless unit certainly implies a common cause, so a found steed would have this qualification as an Ally.
Most steeds has some level of intelligence (comparable to a wolf) so it can share other goals with you too, but a found steed can have more advanced goals (see this question on creature sentience).
Even a normal steed should count as an ally, but a found steed has extra qualifications as an ally.
Is the mount within 5 feet?
There are some cases where your mount won't be within 5 feet of the enemy (such as if you are attacking a creature 5 feet above you and you are at the highest point on your mount), but typical mounted combat will place your mount within 5 feet of any creature you target in melee.
Additionally, attacking with a reach or ranged weapon would not necessarily qualify as the ally needs to be within 5 feet of the creature you are attacking, just make sure you keep that aspect in mind.