1. Install Monit as root:

./configure
make
make install

2. Add monit service configuration to systemd:

/lib/systemd/system/monit.service

# This file is systemd template for monit service. To
# register monit with systemd, place the monit.service file
# to the /lib/systemd/system/ directory and then start it
# using systemctl (see below).
#
# Enable monit to start on boot: 
#         systemctl enable monit.service
#
# Start monit immediately: 
#         systemctl start monit.service
#
# Stop monit:
#         systemctl stop monit.service
#
# Status:
#         systemctl status monit.service

[Unit]
Description=Pro-active monitoring utility for unix systems
After=network-online.target
Documentation=man:monit(1) https://mmonit.com/wiki/Monit/HowTo 

[Service]
Type=simple
KillMode=process
ExecStart=/usr/local/bin/monit -I
ExecStop=/usr/local/bin/monit quit
ExecReload=/usr/local/bin/monit reload
Restart = on-abnormal
StandardOutput=null

[Install]
WantedBy=multi-user.target

3. Enable and start monit:

systemctl enable monit.service
systemctl start monit.service