The game against the current holders will take place at Vicarage Road, with a 7pm kick-off.
Matt Bevans' young Hornets knocked out Southampton 2-1 in front of more than 2,000 fans at Vicarage Road in the quarter-finals to reach the last four.
Amin Nabizada, who has starred in the cup run, has had his good form recognised with a call-up to the England Under-18 squad.
The two sides also met in last year's competition, with Watford going down 2-0 at The Vic in round three – Tom Cleverley was the Under-18s’ head coach on that occasion.
The Golden Boys started their FA Youth Cup journey this season with a win over Oxford United at Kings Langley.
They returned to the Orbital Fasteners Stadium in the next round and saw off the challenge of Crewe Alexandra.
The fifth round saw Watford playing at Vicarage Road, and they claimed a notable scalp by beating Tottenham Hotspur.
Watford have a proud history in the competition, lifting the trophy in both 1982 and 1989.
City have been free scoring in the cup so far, smacking in 17 goals on their way to the last four including 5-1 and 6-1 triumphs over Millwall and Leeds respectively.
The game will be played to a result on the night, with extra-time and penalties if required.
Tickets for the FA Youth Cup semi-final against Manchester City will go on sale on Monday March 17 at 12noon.
Fans can get their tickets from tickets.watfordfc.com, over the phone on 01923 223023 and in-person at the Supporter Services & Ticketing Office at Vicarage Road.
Watford Season Ticket holders can attend for free, but their seats will need to be pre-booked.
Adult tickets are priced at £5 and those aged 18 & under can secure their seat for £1, with all seating reserved in the Sir Elton John Stand.
Supporters aged under 14 cannot attend without an adult (over 18) accompanying them.