We did a lot of research on hiking the Ngong Hills before we went and what we read online appeared to be very untrue, after we hiked the Hills for ourselves. If you are not a citizen or resident of Kenya this can be a very expensive ordeal. When we arrived in Ngong, we located the Kenya Wildlife Service, expecting that this was where we would find a guard to accompany us. Instead, they directed us to the building next door which was we think a Kenya military building. It was full of Kenyan military soldiers. We were told by the commander that we (2 of us) had to be escorted by 2 Kenyan military soldiers that cost 1,500 ksh a piece to escort us. We reluctantly paid with some argument, considering that there were only two of us and two soldiers was very unnecessary. Our total bill so far was 3,000 ksh. We then left with the soldiers heading up to the start of the actual hike, where we again had to pay a fee, this time to the Kenyan Wildlife service. For a foreigner it costs 800 ksh per person. Our total bill ended up being 4,600 shilling for two of us to climb Ngong Hills. It was gorgeous, but the soldiers were definitely not needed. We also saw that every other group only had 1 soldier, so we did indeed pay more than we needed to. We went on a Saturday and there were many groups of students, and other events that were happening along the hills. We felt very safe. If you do not take a guard (I am not completely sure if you can go without one, but I would recommend trying) and are concerned about safety, I would recommend staying on the first 3 hills, because they are very open and for us felt extremely safe.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.