Mongodb – Unable to start mongodb as a service using sytemd

I have the following mongod.service file:

[Unit]
Description=High-performance, schema-free document-oriented database
After=network.target
Documentation=https://docs.mongodb.org/manual

[Service]
User=mongodb
Group=mongodb

ExecStart=/usr/bin/mongod --quiet --fork --config /etc/mongod.conf

# file size
LimitFSIZE=infinity

# cpu time
LimitCPU=infinity

# virtual memory size
LimitAS=infinity

# open files
LimitNOFILE=64000

# processes/threads
LimitNPROC=64000

# total threads (user+kernel)
TasksMax=infinity
TasksAccounting=false

# Recommended limits for for mongod as specified in
# http://docs.mongodb.org/manual/reference/ulimit/#recommended-settings

[Install]
WantedBy=multi-user.target

when I try to start the service using systemd it crashes and this is what i can see in the log file:

2018-01-18T14:20:07.380+0100 I CONTROL  [main] ***** SERVER RESTARTED *****
2018-01-18T14:20:07.403+0100 I CONTROL  [initandlisten] MongoDB starting : pid=6223 port=27017 dbpath=/var/lib/mongodb 64-bit host=atsjenkins
2018-01-18T14:20:07.403+0100 I CONTROL  [initandlisten] db version v3.6.2
2018-01-18T14:20:07.403+0100 I CONTROL  [initandlisten] git version: 489d177dbd0f0420a8ca04d39fd78d0a2c539420
2018-01-18T14:20:07.403+0100 I CONTROL  [initandlisten] OpenSSL version: OpenSSL 1.0.2g  1 Mar 2016
2018-01-18T14:20:07.403+0100 I CONTROL  [initandlisten] allocator: tcmalloc
2018-01-18T14:20:07.403+0100 I CONTROL  [initandlisten] modules: none
2018-01-18T14:20:07.403+0100 I CONTROL  [initandlisten] build environment:
2018-01-18T14:20:07.403+0100 I CONTROL  [initandlisten]     distmod: ubuntu1604
2018-01-18T14:20:07.403+0100 I CONTROL  [initandlisten]     distarch: x86_64
2018-01-18T14:20:07.403+0100 I CONTROL  [initandlisten]     target_arch: x86_64
2018-01-18T14:20:07.403+0100 I CONTROL  [initandlisten] options: { config: "/etc/mongod.conf", net: { bindIp: "127.0.0.1", port: 27017 }, processManagement: { fork: true, timeZoneInfo: "/usr/share/zoneinfo" }, storage: { dbPath: "/var/lib/mongodb", journal: { enabled: true } }, systemLog: { destination: "file", logAppend: true, path: "/var/log/mongodb/mongod.log", quiet: true } }
2018-01-18T14:20:07.404+0100 I -        [initandlisten] Detected data files in /var/lib/mongodb created by the 'wiredTiger' storage engine, so setting the active storage engine to 'wiredTiger'.
2018-01-18T14:20:07.404+0100 I STORAGE  [initandlisten]
2018-01-18T14:20:07.404+0100 I STORAGE  [initandlisten] ** WARNING: Using the XFS filesystem is strongly recommended with the WiredTiger storage engine
2018-01-18T14:20:07.404+0100 I STORAGE  [initandlisten] **          See http://dochub.mongodb.org/core/prodnotes-filesystem
2018-01-18T14:20:07.404+0100 I STORAGE  [initandlisten] wiredtiger_open config: create,cache_size=1463M,session_max=20000,eviction=(threads_min=4,threads_max=4),config_base=false,statistics=(fast),log=(enabled=true,archive=true,path=journal,compressor=snappy),file_manager=(close_idle_time=100000),statistics_log=(wait=0),verbose=(recovery_progress),
2018-01-18T14:20:07.617+0100 I STORAGE  [initandlisten] WiredTiger message [1516281607:617760][6223:0x7f5505f409c0], txn-recover: Main recovery loop: starting at 18/6144
2018-01-18T14:20:07.732+0100 I STORAGE  [initandlisten] WiredTiger message [1516281607:732326][6223:0x7f5505f409c0], txn-recover: Recovering log 18 through 19
2018-01-18T14:20:07.798+0100 I STORAGE  [initandlisten] WiredTiger message [1516281607:798817][6223:0x7f5505f409c0], txn-recover: Recovering log 19 through 19
2018-01-18T14:20:07.863+0100 I CONTROL  [initandlisten]
2018-01-18T14:20:07.863+0100 I CONTROL  [initandlisten] ** WARNING: Access control is not enabled for the database.
2018-01-18T14:20:07.863+0100 I CONTROL  [initandlisten] **          Read and write access to data and configuration is unrestricted.
2018-01-18T14:20:07.863+0100 I CONTROL  [initandlisten]
2018-01-18T14:20:07.876+0100 I FTDC     [initandlisten] Initializing full-time diagnostic data capture with directory '/var/lib/mongodb/diagnostic.data'
2018-01-18T14:20:07.877+0100 I NETWORK  [initandlisten] waiting for connections on port 27017
2018-01-18T14:21:02.288+0100 I CONTROL  [signalProcessingThread] got signal 15 (Terminated), will terminate after current cmd ends
2018-01-18T14:21:02.288+0100 I NETWORK  [signalProcessingThread] shutdown: going to close listening sockets...
2018-01-18T14:21:02.288+0100 I NETWORK  [signalProcessingThread] removing socket file: /tmp/mongodb-27017.sock
2018-01-18T14:21:02.289+0100 I FTDC     [signalProcessingThread] Shutting down full-time diagnostic data capture
2018-01-18T14:21:02.291+0100 I STORAGE  [signalProcessingThread] WiredTigerKVEngine shutting down
2018-01-18T14:21:02.357+0100 I STORAGE  [signalProcessingThread] shutdown: removing fs lock...
2018-01-18T14:21:02.357+0100 I CONTROL  [signalProcessingThread] now exiting
2018-01-18T14:21:02.358+0100 I CONTROL  [signalProcessingThread] shutting down with code:0

but starting it via terminal using the following command: sudo -u mongodb mongod --quiet --fork --config /etc/mongod.conf --port 27017 works. I am unable to figure out why this is happening at the moment so any hints/pointer will be appreciated.
The following is the output from the version switch of the installed mongod:

$:~ mongod --version
db version v3.6.2
git version: 489d177dbd0f0420a8ca04d39fd78d0a2c539420
OpenSSL version: OpenSSL 1.0.2g  1 Mar 2016
allocator: tcmalloc
modules: none
build environment:
    distmod: ubuntu1604
    distarch: x86_64
    target_arch: x86_64

I'm on Ubuntu 16.04.3 LTS