MongoDB systemd service unit configuration

mongodb.service

[Unit]
Description=MongoDB Database Service
Wants=network.target
After=network.target

[Service]
Type=simple
PermissionsStartOnly=true
ExecStartPre=/bin/mkdir -p /var/lib/mongodb
ExecStartPre=/bin/chown mongodb:mongodb /var/lib/mongodb
ExecStart=/usr/bin/mongod --config /etc/mongod.conf
ExecReload=/bin/kill -HUP $MAINPID
Restart=always
User=mongodb
Group=mongodb

[Install]
WantedBy=multi-user.target

This systemd config requires you to specify empty processManagement and systemLog blocks in your config, these aspects are managed by systemd and journald in this case.