In order to grow your hair and beard to full length in Red Dead Redemption 2, you must get Hair Tonic!
How to get Hair Tonic: It can be bought from every General Store, for example in Valentine. It’s found under the “Tonics” category. After buying it, open your satchel (hold 
) and select the Hair Tonic to drink it. You can stack the effect to make your hair grow faster! This means each drink adds a multiplier to your hair growth so drinking 2 or 3 or more hair tonics increases hair growth exponentially. It still takes a while to fully grow out your hair, this happens automatically as time passes.
Drinking Hair Tonics is the only way to fully grow out your hair to maximum length in RDR2.
For more RDR2 guides, check out our full Red Dead Redemption 2 Wiki & Strategy Guide.
It’s not under “tonics”. It’s under “kit.”