@Lenu K
Unfortunately Prior to Ambari 2.7 in Ambari Ui there is no option to see which user has performed the operations.
Ambari 2.7 screenshot:
However in previous version of ambari like Ambari 2.6 you can check the "ambari-audit.log" to see the activities performed by the user.
Example:
# tail -f /var/log/ambari-server/ambari-audit.log
2018-10-04T23:44:21.489Z, User(admin), RemoteIp(101.142.180.147), Operation(Request from server), RequestType(POST), url(http://hd1.example.com:8080/api/v1/clusters/TestCluster/requests), ResultStatus(202 Accepted), Command(RESTART), Cluster name(TestCluster)
2018-10-04T23:44:21.535Z, User(admin), Operation(Restart all components for SmartSense), Status(IN_PROGRESS), RequestId(852)
.