The design and usage capacity of mobile handheld devices have advanced tremendous in recent times, from being used solely for audio calls to the recent incorporation of augmented reality in smartphones. These new smartphone applications are power intensive causing excessive heating in phone parts, primarily batteries and processors. In some cases, the temperatures of phones in use exceed the 45 C threshold temperature of discomfort. These undesirable high temperatures affect user experience and form the basis of ongoing studies to improve thermal management in handheld devices. This work analyzes the thermal profiles of three smartphone models A, B and C for common tasks such as music playing, voice calling, video streaming and 3D online gaming. Transient surface temperature distributions were obtained with infrared imaging and thermocouple sensors, while processor and battery temperatures were obtained from inbuilt sensors, for the phones operating with and without phone casings (test and control cases, respectively). Test results showed that casings generally inhibit the dissipation of the heat generated within the phone, leading to increased processor temperatures. Comparisons between thermal profiles for different phone casing materials showed that plastic casings caused the least temperature difference throughout the test duration, followed by silicone, carbon fibre and leather casings, in ascending order. The rises in processor temperatures during the test duration were compared for the different smartphones: the phone B had the least temperature rise, followed by phone A and then phone C. These gave indications of the power consumption by the phone processors when undertaking the test tasks.