Short answer: yes, it is a modern phenomenon.

Most historians, social scientists, and psychologists who study the mass killing argue there are two defining characteristics:

The number of people killed in a short time period. Most researchers follow the FBI's definition: "[a] number of murders (four or more) occurring in the same incident, with no distinctive time period between the murders" (Morton & Hilts 2006).

The perpetrator's motivation and isolation. Throughout history, there are many attacks by a single perpetrator that left many dead in a short amount of time. For example, during the Second Cherokee war a single Cherokee warrior killed and scalped several families of Floridian settlers in 1835. But we don't consider this a mass killing in the same way as Sandy Hook or the Orlando shooting. It took place in the broader context of war. The Cherokee warrior's motivations were clear and were shared by a large, recognized group. By contrast, most mass murderers are isolated and act without clear motivations.

Mass killings are better defined by what they're NOT: state-sanctioned violence (like public mass executions), religious or ethnic purging (like the Huguenot Massacre of 1565), guerrilla or terrorist warfare (like George Washington's tactics against the Iroquois or ISIS suicide bombings), serial killers (like Carlo Gesualdo), gang or tribal violence, etc.

The first wave of mass shootings kicked off in the 1920s, notably with the Bath, Michigan school bombing. On May 18, 1927, Andrew Kehoe, a farmer, killed his wife and then blew up his house, farm, and the Bath elementary school, killing 6 adults and 38 children (Schildkraut and Elsass 2016). Familicide rates in the 1920s had already bizarrely skyrocketed (Schildkraut and Elsass 2016). The second wave of mass killings started in the 1960s, and has been almost exclusively mass shootings since then.

tl;dr : the kind of mass killing you're thinking of-- lone wolf takes out multiple people for no reason-- has only been around for about a century.

Sources: Morton, Robert J. and Mark A. Hilts. "Serial Murder: Multi-Disciplinary Perspectives for Investigators." Schildkraut, Jaclyn and H. Jaymi Elsass. Mass Shootings: Media, Myths, and Realities. Santa Barbara, CA: Praeger Press, 2016.