We report a transparent 269 nm deep ultraviolet (UV) light-emitting diode (LED) with a thin Mg-doped AlN p-type ohmic contact layer. At 20 mA direct current, the forward voltage is 6.2 V and the optical output power is 11.8 mW, translating into wall-plug-efficiency (WPE) and external quantum efficiency (EQE) equal to 9.5% and 12.8%, respectively. The device maintains 70% of its original optical output power for more than 1000 hours (L70≥1000 hrs) at a current density (J) of 88.9 A/cm<sup>2</sup>. Experimental data support that this device will have a significantly increased L70 for J ≤ 30 A/cm<sup>2</sup>. We also demonstrate that for deep UV LEDs the EQE vs current-density (EQE-J) curve can be well fitted by the standard carrier recombination model (ABC model), and internal quantum efficiency (IQE) and light-extraction efficiency (LEE) can thus be extracted. Furthermore, we propose a method for quick assessment of LED’s lifetime, through fitting of EQE-J curves before and after short-term reliability test.