How to unmarshal objects inside array

The code you presented should unmarshal your JSON successfully; the issue is with the way you are trying to use the result. You say you want to use fmt.Println(data.Lat, data.Lon) but this will not work because data is a slice ([]ResponseData) not a ResponseData. You could use fmt.Println(data[0].Lat, data[0].Lon) (after checking the number of elements!) or iterate through the elements.

The below might help you experiment (playground - this contains a little more content than below):

package main

import (
    "encoding/json"
    "fmt"
    "log"
)

const rawJSON = `[
  {
    "name": "London",
    "lat": 51.5073219,
    "lon": -0.1276474,
    "country": "GB",
    "state": "England"
  }
]`

type ResponseData struct {
    Name    string  `json:"name"`
    Lat     float32 `json:"lat"`
    Lon     float32 `json:"lon"`
    Country string  `json:"country"`
    State   string  `json:"state"`
}

func main() {
    var data []ResponseData
    err := json.Unmarshal([]byte(rawJSON), &data)
    if err != nil {
        log.Fatal(err)
    }

    if len(data) == 1 { // Would also work for 2+ but then you are throwing data away...
        fmt.Println("test1", data[0].Lat, data[0].Lon)
    }

    for _, e := range data {
        fmt.Println("test2", e.Lat, e.Lon)
    }
}